Open source projects on GitHub rely on D6 section of GitHub TOS. That is the part where a user declares that when s/he makes a contribution to a project, s/he does so under repository license. Also that you have right to do so (e.g. when you are on employer clock, your employer must agree).

It's not just a duty of copier to ensure they are granted a license. Legal stuff is never clear cut, but if someone agreed to TOS, it should be generally (that's a funny word that can mean anything) safe to assume that s/he does abide by it.

In this case, there is an obvious conflict and when there are two conflicting clasuses... that's fun for laywers.