AFAIK, unless that tree has signed commits in the history after the commit introducing the cursor files (or it's otherwise verifiable, like having been linked by a member of their team), that's not a smoking gun.
I remember a meme that was shared a while ago, where somebody forked the Linux kernel on GitHub, made a joke commit under Linus's details (which are NOT verified by design), and posted them around. I can't find an instance of that right now, but here's a somewhat similar example, where somebody put a fake backdoor in their fork and changed the url to the original repo, which lets them pretend the commit came from the original repo.
I'd love to see a smoking gun to confirm those claims, but commiting as somebody else, with a fake time, and editing history aren't that difficult - if they could remove the file from history, somebody else could add it to history.