If they get rid of licensed stuff it should be ok no? I really want to use this and seems inevitable that we'll need it just as google translate needs all of the books + sites + comments it can get a hold of.
On the other hand, if only permissive licenses that also don't require attribution is used, well, then for a start, the available corpus is much smaller.
I guess my point is, you can't be positive that even if you're following the license in a repo you forked that the repo owner hasn't already violated someone else's license, and now transitively, so have you.
In fact, that seems to be exactly the problem shown in the tweet - someone copy-pasted the quake source and slapped a different license on it, and copilot blindly trusted the new license.