I would be Happy to pay money to see in a folk club the Original Carter Family ,Leadbelly,Jimmie Rodgers,Blind Lemon Jefferson,Roscoe Holcomb,Clarence Ashley,but not John Lennon.
I may not be able to define what is folk music,but I have my own ideas[Subjective] ,as to what it is in a performer ,and his performance of songs, that makes him a Folk Singer and I do not see/hear that in Lennons work.
Lennon was a skilled songwriter and performer,so was Nat King Cole.
I have never been moved to sing any of their songs,and dont regard them as Folksingers ,neither do I regard myself as a Rock OR Jazz or Reggae singer.