I could never understand why, for example, the Incredible String Band or Nick Drake were considered to be "folk"

Robin Williamson published two books of traditional tunes which made them very accessible. That fitted in with the instrumental breaks in the songs and tied what he was doing to the tradition. For me, the books have worn a lot better than the recordings, which I would not make any effort to listen to again.

I never even heard of Drake until he'd been dead for 20 years (and realized I hadn't missed anything). Did he actually label himself as "folk"?