The unique part of this song is how it glides from minor to major at the end. In classical music, this ending would be called "tierce de picardie" or a picardy third meaning you make the last chord in the minor key song a major. This is probably because Leadbelly never played minor chords. Pete recognized this and incorporated it beautifully with a major sixth chord at the end of the chorus phrase.
Stravinsky was quoted as saying something to the effect that great composers don't plagarize anything, they out-and-out steal.
Sounds like the folk process to me.