Lighter, I'm afraid I don't understand the points you are making. "Casey Jones" and the music of 12th century aborigines may have nothing in common musically. That doesn't mean it is pointless to describe the performers as traditional musicians.

It is quite possible to look at a culture and identify whether a folk tradition exists - it is an observable phenomenon. It is true that a folk tradition may overlap with professional "high art" to a greater extent in some cultures than in our own, which may blur the boundaries.

Whether every piece of music performed within that tradition is necessarily "folk music" is another matter - in a culture which is exposed to other "professional" music then it is likely that these will be performed alongside true folk songs. Over time they may mutate into folk songs.

Surely it's possible to look at a singer, whether they're from Norfolk, from the Appalachians, or from Timbuktu, and decide whether or not they are part of an ongoing folk tradition in terms of their own culture. You wouldn't necessarily expect their music to sound similar, or even be mutually understandable.

Whether or not someone is a traditional singer will depend on the cultural environment in which they learned their craft. In that they are similar, even if the songs they perform are not.