If folk music simply means music about folks, then all the songs about sheep, horses, dead dogs, trains, the seasons, etc, are not folk music. The category then includes anything not performed by horses. Of course, you could probably train a horse to play a bodhran, so Irish music wouldn't be folk...
If folk music is music by or for people, then all music is folk music, so you don't even need to use the word "folk" - just call it music.
If what Bob Dylan sings is folk music, then what Frank Proffit sang is _____?
Find me something to call songs or tunes passed down the generations in a community by oral tradition. It was called "folk" a few decades ago.
I honestly have been through this discussion many times in many places. One thing I don't really understand is why many people find it necessary to apply the "folk" label to music they just happen to like, and/or find meaningful. Why bother using the word? (Perhaps because "pop," which is what I think a lot of it has, in recent history, come to mean sell-out city?)