I have never seen anyone play with their thumb before! I kept expecting the guitar or amp to burst into flame.
I used to be a John Hartford fan back in the 70's but haven't listened to him much since then. I remember seeing a solo concert in a small venue at Carnegie Mellon University, maybe 1976 or 77. He was quite a showman, playing a variety of instruments and also singing acapella. There was some kind of board on the floor and taps on his shoes for doing percussion effects with his feet. The show was a mix of traditional bluegrass and his own strange style. A lot more up-beat than that clip.