Because it's metal?

It's fairly unlikely to be a scene written by Brandon. Brandon's material is still in draft form/ongoing editing, for one thing, and for another Harriet has always previously picked RJ scenes from his original prologue for these readings.

Add to that the fact Brandon has said he's set apart at least one scene from the original and almost completed prologue by RJ to use in each TOM and AMOL, so that RJ's touch would be on the beginning of all three books.

The scene fits all the criterias. It has the feel of Jordan's writing, but like the previous prologue scenes, it's a bit more unpolished than usual, and it's been edited by Harriet, perhaps retouched a bit first by Brandon as well. Jordan isn't there anymore to polish the language of his almost finished scenes, it's mostly left to Harriet, thus the few un-Jordan words or phrasing here and there.

Then, this scene fits perfectly with the themes and motifs of the other prologue scenes spread between TGS/TOM. It forms a coherent whole. The image that people have to make weapons out of anything they find or ordinary objects was also present in the farmer & blacksmith scene published in TGS. The scene is also a but "timeless" and it brings a resolution to a loose end rather then set anything new in motion, which is no doubt why Brandon picked this one out of all the others to keep for the third book. When analyzed a bit, it's clear this scene was obviously RJ's intro for Elayne and Mat's storylines. As a red herring, it hints at strife in the capital. It also introduces the notion that metal now melts. What Jarid planned threatened to divide Andor on itself, also capturing the city Mat is expected to head for soon to free Moiraine, threatens an invasion of the TR which reminds the reader that Elayne is Queen and believes the TR are "in rebellion". Jarid was the last "loose end" of Elayne's victory in KOD - the prologue scene simply revealed it to be a red herring (a device repeated elsewhere in the prologue) and finish to resolve the "Andoran succession" storyline from KOD.

This was all a set up for Mat and Elayne. If we scrap Brandon's "prequel" for Mat (all his TGS scenes), his real first scene, written by RJ himself, was in this tavern in Caemlyn, revealing he had a mysterious letter from Verin, that he was surprisingly already in Caemlyn because Verin made him travel there and warned him the Shadow was after him. Elayne wasn't answering his repeated messages, and that he's desperate to talk to her to get those cannons built... which the prologue scene had suggested might simply... melt, but also it introduces the theme of making weapons for TG, and of acts of creativity being a symbolic defiance to the DO (a subtle but very important theme of AMOL as a whole) which continues in Mat's storyline with the cannons, and Elayne's acts of creation with ter'angreal. Then there was the gholam. That was how Mat was to be introduced by RJ in the original AMOL, with two mysteries, one about Verin's letter, and one about the reason why Elayne was silent, and three threats, the gholam and that Forsaken after him (that also gets confirmed in the prologue, by Graendal who mentions Cyndane and Moghedien are after Mat/Perrin for Moridin) . The prologue scene also set up the themes that would play out in Elayne's pre-TG "climax", e.g.: her deal with Perrin over the TR and so on, and how she unified not only Andor by strenghtening the West, but also claimed Cairhien, her creations of ter'angreal continued the theme of acts of creation etc.

It's a vert classic "multi purpose" RJ scene, a very classic prologue scene for him as well. Brandon doesn't do this type of scenes, he writes shorter single purpose scenes. That one is clearly RJ's own, and it was almost certaintly to end the original prologue as it is a "book end" scene, echoing the other "book end" that started the prologue (the farmer and blacksmith from the Borderlands, with the blacksmith leaving everything behind to go north and face "the storm", making weapons there) and which was far darker/threatening/desperate. That scene introduced the theme of forging weapons too, but this time it was a blacksmith and RJ's very subtle foreshadowing for Perrin and the forging of his hammer and the symbolic power of blacksmiths and their role creators/leaders, one of his main pre-TG climaxes/resolutions. It echoed subtly the ending of Perrin's KOD story, with him finally resolving his obsession with Faile now she's rescued, and getting his priorities straight again. The first scene was also an intro for Lan (which would continue with a real short Lan scene later in the prologue echoing the same themes as the first scene), then a scene was the first intro scene for Rand's storyline, about the capture of Semirhage and a reminder he was heading for Arad Doman and still sought a meeting with Tuon. Tylee's scene then concluded the intro for Tuon's storyline - and in the light of Rand's intentions it ended with the apparent death of the sole Seanchan who saw an end to the conquest and an alliance with Perrin and co. as the right decision...

Graendal continued Rand's intro this time by introducing the threat he'd face in the Arad Doman storyline, completing this with a short intro for Egwene's storyline (Mesaana in the Tower, saying she's ready and so on), and Demandred. Brandon split/rewrote that scene (or even added a "prequel" of a sort to it). He put a rewritten first part in TGS where Graendal receives her order about Rand (which may not have been planned by RJ - she could very well have been shown returning from that meeting and allude to all of this in her head as she conversed with Aran'gar), and kept the second part that feels more like RJ in the TOM prologue, and that one resolved the issue of where Aran'gar now was and her allusions about using Graendal's agents and killing Rand from the KOD Forsaken meeting, further embroiling matters for Rand in Arad Doman. Itulrade then finalized Rand's intro by deepening the problems he'd face in AD, and concluded his own series of prologue scenes, again in a fairly dark manner - an expected suicidal battle, that he intentionally baited the Seanchan into a war with Arad Doman he knew he couldn't win, and to end it all in glory.. and in certain death. Sheer madness, and a subtle intro to themes of darkness and madness that would surround Rand early in AMOL, and drive all the storylines toward dead ends and madness and despair too.


Then on with Perrin and a TAR scene with Hopper, and the conclusion to Galad's series of prologue scenes, that complicates matters for Perrin's storyline and has Galad arrested by Asunawa... Galad meeting Perrin didn't look so bad, but Asunawa... And we had a dark scene in Kandor, continuing the intro to the invasion of the Borderlands, adding tension/urgency to the farmer/blacksmith scene and the Lan scene. Then it was Padan Fain, one more important player re introduced, and completing the elements of the Moridin/Graendal scenes. There were two scenes that had "rays of hope". The first one introduced (as it's been done in Mat's storyline with the new Bayrd scene) Cyndane or Moghedien was after Perrin with Masema's "false Rand" - but like the Jarid scene it resolved a long ongoing red-herring of RJ: Masema as a threat. It started with Masema planning stuff that would have been bad and would have thrown him in Rand's AD storyline by bringing him to Almoth Plain and then... surprise, Faile killed him. Finally there was the new scene with Jarid. Again a false threat of splitting Andor and perhaps clashing with Perrin if he was heading for the TR and then... surprise, Jarid is dealt with right away, and the prologue that all started threateningly with the "storm" and TG starting as Rand, Perrin and Mat were embroiled in so many complications ends with a big cheer and call to battle and defiance by a soldier and his primitive weapon, but all too willing to face the Last Battle for his land and humanity (echoing in contradiction the despair in the middle of the prologue, with the suicidal intents of one of the great captains) . That's the spirit... And on to the book.

It seems to have been a really well balanced prologue originally, setting the main themes and story points really well. It may not be the whole prologue (and the scene order is almost certainly not RJ's original one). There may be other missing scenes which are in AMOL, and Brandon said once he moved prologue scenes into the book and back etc.

However it suggests strongly Jordan planned to start with Egwene / Mat in Caemlyn as her storyline is the one missing altogether in the prologue, then Perrin's storyline for which he had to go futher back than the others. And gradually he would have woven in Rand. To further suggests that, Jordan had very advanced Egwene's storyline (BS said there was more RJ than him in Egwene's scenes) and Mat's (again, BS said there was more RJ than him in Mat's TOM scenes), but had mostly just outlined Rand's and Perrin's except for a few key scenes (dixit Brandon).

Each time he'd switch between the storylines, he would have reduced the timeline gaps between them (Mat and Rand advanced more slowly time-wise, Perrin and Egwene had ellipses), until it all came together with Rand's epiphany and the other synchronized pre-TG climaxes, Egwene winning the Tower after the Seanchan attack and defeating Mesaana as Perrin faced Slayer which brought the downfall of Graendal on the side, and then he forged his hammer, heading to the Tower of Ghenji for Mat. Then the "merging" began in the aftermath, with Perrin meeting Elayne, Mat rescuing Moiraine, Rand meeting Egwene.