"Anyone" would include Demandred too... why would a Forsaken be more likely to know stories loosely based on facts regarding Mat he barely ever discussed with anyone like the truth about the hanging etc. than people in Caemlyn? If you ask me, Demandred in particular seems not to give a damn about Mat and Perrin. Demandred's nemesis has always been LTT/Rand. He's preparing for TG and is focussed on Rand. I don't think it's even occured to him at this point the general he might face most during TG is Mat rather than Rand, that Rand might suddenly step back and take a secondary role. This is an advantage for the Light. Demandred knows LTT from top to bottom, and he's updated his knowledge of the enemy by closely watching Rand. Mat is... full of surprises, not one general but hundreds at once, and so virtually unpredictable. I seriously doubt Demandred paid nearly enough attention to Mat to be aware of this. From all his observations, Demandred is probably prepared to face Rand who as far as he can tell, is in matters of strategy/tactics was usually a pale shadow of LTT, with occasional surprising flashes of LTT's genius and an infuence from competent third age generals around him... and all the luck of a ta'veren.

Rand has almost always been discreet when he involved Mat, it's quite possible Demandred isn't aware of Mat's role in elaborating some of Rand's strategies. If he cares about Rand's understaff, it would be about the visible ones like Bashere, Dobraine, Rhuarc and the Aiel, Itulrade. Those have been Rand's military helpers since LOC or so. They are all more likely targets for Demandred to seek to eliminate than Mat, because Demandred's spies (or Mesaana's/Semirhage's - with those three it doesn't massively matters) would be fully aware Rand relies on them, and they've truly helped him. Not Mat. Only Bashere knows how pivotal Mat was in designing the plans against Sammael. Mat has a fair chance to remain an ace in Rand's sleeve. If anything, Mat has been dismissed as an irrelevant buffoon by Demandred, the most likely first hand account of Mat he could have gotten is from Semirhage. As far as we can tell, Semirhage never figured out Mat's role in engineering Tuon's disappearance/escape. To her he was a total idiot, the sex toy of the Queen. As for the Band, there's just a bunch of non-channelers to a man like Demandred, their exploits irrelevant as facing even Taim's men they would be shredded in little time. Same for the Aiel... the Shaido broke completely facing a small group of Asha'man...


How Guybon knows this stuff about Mat is fairly simple, but first he's highly placed in Elayne's Guard and it's his job to know the rumours floating around the city, especially those concerning the commander of a mercenary group named after an army of legend, and who seems to have quite a reputation. That explains why Guybon remembers/paid attention to all those details. As for why he knows them in the first place... Olver knows and he is highly impressionable and to him Mat is a hero of legend. Olver is always around the soldiers of the Band, many of which are just as impressionable as he is, and worshipping Mat as a hero as much as Olver does. Soldiers gossip and naturally they would spread tales about their legendary commander in the taverns/inns of Caemlyn, especially to the patrons who gossip about the Band of the Red Hand and hav their fact wrong. So Master X claims for e.g. the Band are Murandian spies and their commander is a bandit they freed from the gallows. Soldier X interjects Lord Mat is a hero who's (insert aggrandized episode about his military prowess, insert garbled version of Olver's already garbled understanding of what really happened to Mat... and add to the mix all the informers Guybon's boss, Birgitte, has in the city and you get tons of reported rumours reaching Guybon's desk about Mat Cauthon.

As someone else said, Guybon is among those ruled out as being a possible alter ego of Demandred. In any case, if Demandred uses Guybon's appearance (which aside from Lanfear very briefly and Mesaana who used a young loner non one knew very well and who didn't have to remain around other AS very long, the Forsaken have never risked impersonating existing people, rather inventing people and using pseudonyms) we never got a scene so far where it was Demandred impersonating Guybon (or anyone else). Personally I'm not sure why people are so obsessed with finding an alter ego for Demanded after Jordan revealed we had not seen him using an alter ego by KOD... Isn't it painfully obvious Demandred has simply not followed the strategy of so many of the others of attempting to infiltrate himself into an existing group? Demandred's remained unseen, safely behind any front line, building his forces for TG. Any move he's made/plan he put in motion, he most likely did it using proxies. Another clue is that the only DF (Ashaman) we know to have dealt with him directly have apparently met him as Demandred. I think Demandred has had fairly little contacts with non-DF, and with his followers he most likely never bothered to hide his appearance or name, because with the kind of master plan he's likely to have hatched (armies to lead to TG, a variation on that), not hiding to his underlings that they serve under the great Demandred, best general of the Shadow in the WOS - that they have been chosen by Demandred, is all to his advantage.