Putting aside, for the moment, the "oral tradition" aspect of folk song, there's still a recognizable--if difficult to define--idiom that's been generally accepted as "folk song". Some singer-songwriters tend to produce material that's in that idiom--Art Thieme, Woody Guthrie, Shel Silverstein (at times), Steve Gillette, Pete Seeger, Ewan MacColl, to name a few. Others tend to stretch the limits of that idiom--Martin Carthy, Fairport Convention etc. but retain some clear ties to it. And still others have departed so far from it that it's difficult to justify their inclusion in the designation.
    I don't care what horses do or don't sing.