Jim,
I think you've put your fingure on it nicely. However, regardless of whether or not Beethoven was a genius, it wasn't because it could still write music even though he was deaf. We all hear music in our heads, yes? Beethoven could, and had the ability to write those sounds down. Have you ever seen a musician humming to himself, then writing the phrase down on MS paper. Arrangers do it all the time. No big deal.