I ignored the irrelevant and unsupported anecdotal premises and focused on and defeated the underlying argument which is "Paul McCartney is a great musician because his memory is freakish." I think, instead, like anyone at the top of their field, McCartney is a great musician because he studied and practiced music.

There's plenty of people who studied and practiced music who didn't become great musicians. So, it's a necessary, but not sufficient component. I suspect great memory is absolutely a huge boost for any musician, probably even more so than in other fields (where it's also a huge boost).