But the thing to note is that a user can have a right to distribute (as with GPL) but does not necessarily have the rights to the license.
So if the user uploads the source to GitHub, they agree to the terms (which they may not actually have the rights to) but that isn't equivalent to the rights owner giving GitHub the rights to distribute the source under a different license.
The TOS can only modify those distribution terms (if it even can be found to be legally binding) if the user uploading the source is the rights owner which in so many cases is not the case.
I think the bigger question is whether GitHub will be able to honor DMCA requests that pertain to copyrighted materials showing up in Copilot's suggestions.
Are we entering in to a new realm where a DMCA (or DMCA-like) request can be filed to remove content from the training data for an AI (and likely cause it to require retraining)?
So if the user uploads the source to GitHub, they agree to the terms (which they may not actually have the rights to) but that isn't equivalent to the rights owner giving GitHub the rights to distribute the source under a different license.
The TOS can only modify those distribution terms (if it even can be found to be legally binding) if the user uploading the source is the rights owner which in so many cases is not the case.