John had a very distinctive "guitar player" style on banjo; I am convinced he did all the banjo work on the albums. His playing is almost exclusively dominated by forward rolls. That's not a style that a bluegrass player, or pro-banjo player would use. ("Low Bridge" is all forward roll.) In the studio, they would record the instrument tracks first; then overdub the vocals; then overdub "sweetening" vocals on the choruses. My point is that the instrument tracks had to be perfect before they added any vocals. John would have plenty of time to get it right.