Interesting.
I think it's probably real. There's too much going for it, and last game's ESRB leak proves that you can try to explain away all sorts of supposed inconsistencies and still end up having the thing you're trying to disprove be real. It does contradict Vergeben though, which is interesting.
It's a shame that Zelda gets shafted once more, but not entirely surprising, and there's too many great, heavily requested characters on here for me to get too upset about Skull Kid being missing. I mean, a game with Ridley, Isaac, Geno, and Banjo as base newcomers? Are you nuts? Sakurai must be.
In the end, I would be disappointed not to see Skull Kid, but not too terribly surprised. And assuming he's not a boss, he's still completely eligible for DLC later on!
And now for my obligatory warning: treat this as real. Convince yourself that Skull Kid is not playable. That way, if he's not, the blow is softened, and if he is, it's a fantastic surprise.