Rumor has it that GitHub has a flat namespace for commits. They don't store "user1/repo1/abcd1234" in one file and "user2/repo2/abcd1234" in another. Both references point to the same commit in a global shared space. If the hashes are truly unique, then that never matters, because the odds are approximately 0.000000000...000 of you and I accidentally generating the same commit. However, if I see that you pushed commit abcd134, and then I can build and push a colliding commit, and the backend doesn't check uniqueness before writes because the odds are infinitesimal that it'd ever matter, than voila, I've updated your repo by writing to my own.

Or if first writer wins, and I know that you have a popular non-GitHub repo that you're about to migrate into it, then I could pre-poison the namespace by writing my own version of a commit that I see you already have in Codeberg or Savannah or wherever.

I don't swear that this is how GitHub actually works, but I've had knowledgeable friends swear up and down that it is. And honestly, it'd make sense. They could shard storage by the first 4 digits of the hash or something, and that'd be vastly more efficient if all commits were writing to the same space.

I'd expect an LLM to prove this (if true) in ~ 60 minutes, given just your post and "try to prove this true or false; here's a github PAT".