The GitHub ToS is not a software license. It is the terms for using the GitHub service. The penalty for breaking it is that your GitHub services might be terminated, not that they can somehow relicense your software.
> If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking).
> you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking)
So, on GitHub, we can "use", "perform", and "reproduce". Does editing/modifying fall under any of those verbs?