Alas, that guy is totally unreliable, one of the least reliable WOT fan around.

His previous reports have featured BIG errors, contradicting what Brandon had said at previous signings (he's the one who reported that Brandon had gone on at length at his signings about Perrin and Mat's groups meeting and merging for good in TGS. Someone else rather said Brandon had RAFOed that question about Mat/Perrin - merely adding they were roughly in the same area at KOD ended, and much later in the signing had spoken of the main storylines merging at the end of TOM for TG....).

About Sulin, I would take it with a grain of salt until someone reliable reports Brandon saying that too. With Dida, there's a 50/50 chance he has it wrong! He speaks as fact about Elayne's prophetic dreams, Hurin's foretelling and that sort of misinterpretations, so that gives you an idea....

It's quite possible they discovered in the end they blundered, but the whole comment about needing Maria to convince him about Sulin is a bit fishy (Dida also tends to embellish his stories). The thing is Brandon never argued the fact Sulin was with Perrin at least up to the aftermath of Malden, he told people a few times he knew very well she was with Perrin and the fact she was with Rand later was not an error but a RAFO. IRRC (but I'm not sure I remember it right), he had also answered that personally to Jason of DM when he asked (before the book came out).

It's perfectly possible (perhaps even likely) Brandon made a mistake about Sulin in TGS and while on tour imagined there was a plot reason behind this when in fact there's none, but as long as only Dida reports that, I'd wait.