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,

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.