But this is the same with coding. The reason that AI can write code from a description that is shorter than the output is in large part because it makes decisions about the behavior that were unspecified in your prompt. We accept this for coding apparently, I guess because those decisions are often unimportant. We might accept it for writing too. I hope not.

Not necessarily. Poorly worded, ambiguous, confusingly ordered writing can be massively improved without changing the core content. Better setups and explanations can be longer without changing the message or meaning.

Look at it the other way, could you take a good longer message you’ve written and make it shorter and less readable for your audience while still making sense to you and containing the key points?

wasn't it thoreau who said "Not that the story need be long, but it will take a long while to make it short"

personally, I think there's a time and place for short versus long, just like there's a time and place for a 45mins TV episode versus a 2 hour marathon movie.