Playing every instrument with god-like ability & accuracy has nothing to do with arranging music. You can read music & play the notes perfectly. There's plenty of savant/robotic music players out there without a creative bone in there body.
You have a point
I mean, I consider myself an artist as well as a musician, but I've met great pianists, singers etc who can't write or compose. Maybe if it is specified that said ability comes with the creativity associated with artistic ability.
Beyonce may just be a singer and performer but she at least showed her capacity for understanding artistry when you look at how well put together something like Lemonade was from a full artistic perspective - visually, audibly etc. Singers who are technically better than her and can play a bunch of instruments probably wouldn't be able to put something like that together. There are millions of pianists who can play an illustrious Beethoven piece, but there's only one Beethoven who can compose such a piece.
There's a certain understanding of dynamics when it comes to composing and even performing existing music that is different from just learning how to play a piece. I think in essence, neither you or poitier are wrong - just coming at it from different angles.