The argument is set out above that "folk" is a matter of style. It is not. At the theoretical level (1954 or Karpeles definition) it is a matter of derivations.

If it were a matter of style then if English traditional song could be folk music then there could be no Chinese or Russian folk music - because it sounds different.

Further, if it were a matter of style then when Fairport Convention or Steeleye Span or Blue Horses did a folk song then it would no longer be a folk song.