This is simply not true. Look at Beethoven's manuscripts for instance.

https://guides.loc.gov/beethoven/manuscripts

That's one of the reasons why he spent several years to write a single symphony.

Wow. Can we even be sure we're listening to the right thing? Is it actually possible to read this unambiguously or is there an element of context when reading music, similar to how if you're reading prose the next word is probably grammatically correct and makes sense?

The publisher was generally familiar with Beethoven’s writing and conventions. He’d prepare galleys that Beethoven would proof (and frequently edit). A substantial part of Beethoven’s known correspondence concerns corrections to galleys (and managing payments).

Exactly. The context makes it all pretty clear. Music has its own grammar, and particularly music of the common practice era from about 1650-1930.