IANAL so I don't know whether it is a real problem, a hypothetical
problem or not a problem.
My guess is that it can't affect the main notmuch license as the bulk of
notmuch's copyright is owned by people not responsible for the github
mirror (*), but it might mean that those of us responsible for the mirror are
illegally posting notmuch's code.
Does anyone have any views on this, and whether we should carry on
maintaining the github mirror?