To add to this, the lyrics during the section where Johnny plays are about four traditional fiddle songs that aren’t played in the song itself: Fire on the Mountain, The House of the Rising Sun, Ida Red, and Granny Will Your Dog Bite. I think in the same way that the lyrics of Tenacious D - Tribute make it clear that Tribute is a representation of the greatest song in the world and not the greatest song itself, the music we hear from Johnny’s section is supposed to represent but not be the music he played to beat the Devil.