A performer could write a ballad in a traditional mode, invent a provenance and the vast majority of the audience wouldn't know it wasn't original.

I'm sure that's true, but it's got nothing to do with whether there is a difference between songs that are traditional and songs that aren't. I used to think a friend of mine did a lot more Hank Williams songs than he does - he'd been writing his own without telling me (or the audience). I'm sure the vast majority of the audience didn't know his songs weren't Hank's - and if he'd lied to them about it they wouldn't have been any the wiser. They still weren't written by Hank Williams.