"Especially after watching a special a couple years ago where McCartney proved, on film, that he couldn't actually write music but rather, had to dictate how he wanted the music to sound to an actual composer who was able to score it for him.
I just sat and laughed to myself.
What made it doubly amusing was that he had already been knighted for his so-called musical skill."
The joke's on you.
McCartney's father taught him music. He had piano lessons as a child. Plus plenty of other music classes in school.
He can read and write music.
He chose/chooses instead to play and compose "by ear." (Something a lot of jazz and folk musicians have often preferred.)
McCartney was by far the most accomplished musician of the Beatles. And certainly the group's most sophisticated composer.