I know we've been here before, but now we are minus Jim Carroll, I will dare to venture the opinion that the informal definition of Folk, is some distance away from the academic definition, (such as it is).
Folk, to most club going people, covers Traditional and Contemporary, and that'll do, end of argument. It's just for convenience sake. No amount of hair splitting, is going to bother them at all, and usually provokes a mass exodus to the bar, where they will probably start their own sing around, with impunity. (Pinch of salt required for this bit of imagery. You know what I mean, I'm sure...)