The above is not quite true - McCartney had help from several '
classical'
composers in orchestrating
Standing Stone and in making sense of the music generated by the computer - because while a computer program can generate a decent approximation of
sheet music, it produces clunky, badly-notated music. It's like the difference between working in
Front Page and writing your own
HTML.
Similarly, on his earlier orchestral work Liverpool Oratorio, McCartney had the help of Carl Davis, and on the Beatles' records George Martin orchestrated.
In other words, McCartney lacks a vital skill he needs to realise his work. Luckily, he is good enough that people are willing to do this grunt work for him, but most people aren't Paul McCartney.
Having a working knowledge of the tools of one's trade is never a handicap, and the fact that it is possible for a genius to create good work without the use of those tools does not mean they couldn't have created even better work if they knew them. Nor does it mean an average-to-good craftsman could do the same if they didn't have the required technical knowledge.