If you study music properly learning to 'hear' what you're reading is part of the process. Some people get very good at it. I can follow a score, but I haven't had enough practice to do the sonic equivalent of visualising it completely.

It's a little different for composers because music of that period was constructed according to fairly rigid rules, and as long as a composer followed the rules something listenable would fall out.

Scores were more written rather than heard, if only because most people couldn't afford to keep a symphony orchestra or string quarter in the house. Usually a composer would try out lines or chord sequences on a piano or some other instrument to sketch out an outline, but the orchestration, elaboration and arrangement were all done on paper.

Doing everything mentally was a step, but for someone with thirty years of experience it wasn't as huge a step as it might seem.