Why Brandon (in particular) considers Jordan an impressive master of foreshadowing is because of how he managed to pepper the series with allusions to things that play out much later without making the books predictable or even those elements stand out too much (he achieved that in part by putting so many of them that few stand out as more than casual remarks or observations...). He was apparently amazed by how many elements set up or foreshadowed especially in the first three books play out through AMOL.

The theories get wrong not because there's no foreshadowing or the plot develops "randomly", but because there's tons of it (including several red herrings) and Jordan was an expert at including foreshadowing elements that could be interpreted in a dozen or more different ways (he built his prophecies, which are also a form of foreshadowing, exactly the same way). The theories are most often "off" or largely off, because the foreshadowing is misinterpreted, or because it's only after the fact that the foreshadowing becomes obvious as foreshadowing.

Some of it wasn't overlooked. For instance, the casual mention to Min by Rand that he would rather give up an arm rather than cause harm to her has been spotted as suspected foreshadowing long before KOD when Rand put his arm in harm's way to protect Min, and lost his hand. It had even been linked to the viewings of Elayne "an axe, and a bloody hand, not hers" in a theory that Rand would lose an arm or an hand (good example of misinterpration. The viewings had nothing to do with that, obviously). It was even suggested in a post-COT theory that Rand might be forced by Semirhage to torture Min and would stop it by cutting his own hand.

Another example of misinterpretation: if you remember, I was using the scene in LOC as a basis for my speculation that Semirhage had been ordered by Moridin to capture Rand. That part turned out true enough. The second part was that maybe Semirhage had been expected all along to be captured, that Moridin had been willing to sacrifice her if he must, to capture Rand. That turned out untrue, because it's only in TGS that the apparent sacrifice was decided and happened. Still, we had the part about foreshadowing from the LOC scene right, it's just that nobody had been able to interpret it correctly and derive a theory dead on the money.

Foreshadowing abounds in WOT (there's more in it than in most series I've ever read), but its purpose isn't to make the plot predictable or even to help us puzzle out what will happen before it happens (Jordan started using heavy foreshadowing long before he even knew his books would spark theories, before the internet phenomenon too), it's rather one more element RJ used to give the feeling, after something does happen, of "déjà vu", to give the impression it's all part of a pattern, in which things repeat in variations. Foreshadowing is meant to go largely unnoticed or just subconsciously be absorbed, until after the fact you muse "hmmm... there had been quite a few allusions before to severed hands, no?".

Some people have the nose to spot possible foreshadowing long before the events foreshadowed play out, others are extremely good at making the connections to old foreshadowing as they read a new book the first time, for others they notice a lot of it in rereads only. Apparently, there's even some who deny it's there