Ahah, the second and third links on the page are to GitHub

Well it seems to be intended for repos with large blobs like video games so it kind of makes sense that their own pure source code is still managed with git.

Epic's Unreal Engine development is made in Perforce, and then it was mirrored to git/Github. Or that's how it was a couple years ago. That's kind of expected, VCS is one thing, a forge is another thing.