You are correct. Github's license terms are a hedge against that sort of thing. They are basically saying "the person making this source available is granting the license to d these particular things" - therefore, the user making the source available takes on the responsibility for making sure they actually have the authority to make this source available. If they do not have this authority then they are the ones that will be sued - not Github nor any users who cloned the code.

I've seen people forking Unreal Engine on GitHub without any restrictions for public view. Makes one think about this kind of thing.