Aside from the bold (which is a possibility, could be a second/third hand leak)... you're also disregarding the fact he posted the roster just before E3 and got 3 out of 6 right: they were announced a few days after the leak. He could have said Palutena, Chrom or some other more obvious characters, but he said WFT and Villager (basically disconfirmed by Sakurai himself long ago) and got them right, in a matter of days. You're also disregarding the fact he leaked to a website instead than to the usual forums and message boards, as if he feared repercussions if his identity was outed, because he knew he was right.