Earlier quoted context omitted.
I'm sure that he heard his majestic music in his head as he was composing it.
Me too, but that's a fundamentally different experience. I think the question stands, and I'm interested in the answer.
1. He was wildly successful before he started going deaf. Like performing for and requested by the kings and emperors all around Europe successful. Like graduating from being apprenticed by Hayden successful.
2. He had played and composed a truly insane amount before losing his hearing. As a modern analogy, his father “Tiger Woods”ed him. He had all-day and all-night kind of regime from the time he 4 years old. He had famous teachers and was forced to perform in front of the nobility starting at the age of 6. He was pulled out of school as soon as possible to spend all of his time playing. His life was music, in a way very few humans ever experience anything.
3. Classical music is structured in a way that, it is not uncommon for composers to put the whole piece together on paper before hearing it. In composition exercises, pieces are critiqued without them being played. Exercises even at the highest level don’t need to be played to be judged. Pieces are often submitted and judged without anyone hearing them. It can be hard for someone not used to classical music to understand this.