I'm gonna be honest and say for a split second I thought it was Riku. I was way too eager and it probably clouded my perception. That and it didn't help that the stream was pretty laggy for me.
What do those "fans" mean is that Riku? Riku doesn't look like Xehanort at all.
They mean exactly what they write lol They think it is Riku! I mean, for many of us on this site, we can easily say "oh yeah, that's not Riku! That's Young Master Xehanort!" but remember, many people have only played KH1 and KH2 and in that case, there are only 2 Caucasian males with long silver hair: Riku and Xemnas. Who comes more readily to mind? Riku. So I definitely see the resemblance to those making a quick association.
When the first BbS secret ending at the end of KH2FM came out and it just showed a boy who looked like Roxas, a man with brown hair, a girl with blue hair and Mickey Mouse fighting a bald guy, many people thought Terra was grown up Sora and Aqua was grown up Kairi, me included. With little info, we make assumptions where we can.
My problem with kh ignorance deals with the spread of false information. I can understand people not knowing all the characters(that's ok gaming is a luxury), but I can't accept people who haven't played/watched all the games and profess to know everything.
That is just their fault, after KH3 comes out and they know basically nothing, they'll just get pissed off that "SE didn't make it fair for them," and to that there is nothing we can do but move on. Those are generally the people who always have to be right and trust me, no matter how they have to twist their words, in their mind, they are always right.