"but Folk would be probably any song which has been sang in Folk clubs since the revival."

That's as good a definition as any Betsy, and better than most. The thread title is just another chance for pedants to air their definition phobias, most people have made their mind up about what the F-word means.
The old definitions really do defy logic, so there's no point applying deductive reasoning to them. A 15 verse, tightly rhyming song with a specific tune and choruses that doesn't carry the imprint of a single person? Do me a favour. Valorised for the simple fact that we don't know his/her name? What's that all about. And how come that's folk? I think we should be told. Properly.