A Platonist who thinks the Form of the Good is revealed by mystical insight is a cognitivist and I don't know why you would think otherwise. Wikipedia:) "Cognitivism is the meta-ethical view that ethical sentences express propositions and can therefore be true or false".
Or you're not using standard terminology, in which case, see above.
There seems to be a widespread impression that the metaethics sequence was not very successful as an explanation of Eliezer Yudkowsky's views. It even says so on the wiki. And frankly, I'm puzzled by this... hence the "apparently" in this post's title. When I read the metaethics sequence, it seemed to make perfect sense to me. I can think of a couple things that may have made me different from the average OB/LW reader in this regard: