CoreOS is a fork of ChromeOS, not Gentoo. Both just happen to use emerge. It might poach some Gentoo packages to save work (I don't recall off hand) but build and usage is very different from Gentoo. Importantly, emerge is build-time trivia and has no bearing on the final product; there is not even an emerge binary in a CoreOS image.

(I've seen this meme before, with people - not you - dismissing CoreOS because it's "based on Gentoo," due to the presence of emerge in the build. Pass this clarifying point on to your friends.)