I think YOU in the second line is referring to the listener. But YOU in the first line (if the LSD theory is correct) would refer to Paul. Now that I've said that it does sound odd that he would address Paul and then the listener.
Another theory would be that "Sgt Pepper took the public by surprise because they underestimated Paul/The Beatles". But this doesn't really add up to an attack on Paul as it's more of a compliment so I don't know.