I'd like to see that theory tested in court. Section D3 of the terms says:
> If you upload Content that already comes with a license granting GitHub the permissions we need to run our Service, no additional license is required.
and section D4 does not mention any permissions that GPLv3 does not already cover. GitHub automatically recognizes when a repo is GPLv3-licensed, so it cannot claim ignorance of what GPLv3 is.
If you can't actually grant that separate license, you're misrepresenting your ownership and license to that code