I have also never heard of “public code” being used in that way.
What about people forking/mirroring your code? Or people merely contributing code? There is no one-to-one correspondence between copyright holders and Github users.
Copilot should just comply with the license, that's it.
It would be a lot less trouble for everyone if it was just a per repository setting.
Of course, someone else can still upload your elsewhere-published code to GH. You cannot win.