Comment by ahpook

That's correct, it's doing a clone into an empty repo rather than using the fork API - code is here: https://github.com/github-community-projects/private-mirrors...

As it pertains to the post, since that private mirror is disconnected, none of the concerns about accessing deleted data apply.

The downside is that you don't get any of GitHub's performance and UI affordances from the fork network. But for the use case of private iterations on work headed for a public upstream, that's a trade-off that seems worth making.