With Beethoven there’s the mystique around his deafness: that he composed in spite of this. Part inspiration and part miracle, he could choose the right notes and make them beauteous sound.
But Beethoven could most definitely (and did) compose in his head well before the height of his deafness. With the rigorous work and study, surely into the stratosphere of the Gladwellian ten thousand hours, it would only be natural that Beethoven could work out his music mentally.
It’s like a grandmaster chess player going blind later in his life and yet still being a chess champion. High level players intensively sift through past games, problems, and strategies. Sooner or later a player’s memory works in such a way that a game can unfold without a chess board. Therefore a blind grandmaster doesn’t seem as farfetched.
Though the crux here is that there was intensive work before the permanent alteration. It wasn’t someone just pulling sound out of the air. Would Beethoven have composed the Grosse Fugue if he was deaf at an early age? Would he even be composing? Probably not.
And yet would Beethoven have composed the Grosse Fugue if it weren’t for his deafness? That’s even a more unanswerable question.