" If they did allow that, and retained no control over the forked copy, now you can ride a coach and horses through the access control to a private repo by simply forking it when you have access. "

Actually that's not really true, since your access to the original repository could still be revoked, and you are left with what you got.

Further, see sibling comment.

"My guess is that forking a private repository is a feature github intended to be used where employees or contractors of an enterprise want to fork their employer's repository as part of their development activities for that employer."

what you describe is "internal visibility"

https://docs.github.com/en/repositories/creating-and-managin...