I don't think it could have worked anyway, given https://trufflesecurity.com/blog/anyone-can-access-deleted-a... . The best they could do is hide new test material, and then only if the private repo is kept distinct and isn't recognized by GitHub as a fork.
They’re actually not, apparently it’s “more of a joke” (https://github.com/tldraw/tldraw/issues/8082#issuecomment-39...)