In the end, it's all about tradition. The "New Left" musicianship/culture-bearers (Guthrie, Seeger, Leadbelly, Lomax, Asche, etc.) used a type of folk music (Mainly re-hashed workers songs) as a tool for social change --- and the Dylan's, Cohen's, Och's & Baez's turned that movement into a hugely popular strand of music.

I think people often misconstrue the meaning of folk with the general core of the movements of the 60s. Yet, folk music is essentially ANY carried-on tradition of music... folk doesn't necessarily denote any single musical form.

In truth, there's very little music that truly escapes the realms of "folk music" as any playing style, lyrical manipulation or imagery that is carried down from musical tradition. Every music has a precursor.

But Prince is hugely mistaken. His music, playing-style... even persona... is manifested in tradition.