I agree that he has always hammed this up a bit — and that he has always had a very sophisticated understanding of melody, harmony etc — but I think the key is that he essentially plays by ear. In choirs, I’ve noticed a division between those who look at music and instantly hear it in their heads (from formal training) and those who rely on listening. Both approaches can work effectively but they are different. Also, as you say, it is a good defence mechanism — he didn’t want Jane Asher’s mother to teach him to sight read because he suspected he would not be very good at it,