I think it’s the gh-pages branch on a private repository.
If you look at Nat’s account right now [0], he doesn’t even have the telltale “GitHub Staff” badge, probably because his hiring is contingent upon the deal closing.
So he’s definitely not an admin at the system level, which just makes him a normal account.
Wait, is that a one-off custom landing page? Something that normal users can't do?
I don't see a "hello" repo in his list of repositories: https://natfriedman.github.io/hello/
Can private repos have publicly accessible pages?