I think you've misunderstood the objection.

Lets pick something concrete. It's a medieval script, it opens with two knights fighting. OK so later in the script we learn their characters, historic counterparts etc. So your LLM can match nefarious villain to some kind of embedding, and doubtless has trained on countless images of a knight.

But the result is not naively going to understand the level of reality the script is going for - how closely to stick to historic parallels, how much to go fantastical with the depiction. The way we light and shoot the fight and how it coheres with the themes of the scene, the way we're supposed to understand the characters in the context of the scene and the overall story, the references the scene may be making to the genre or even specific other films etc.

This is just barely scraping the surface of the beginnings of thinking about mise en scene, blocking, framing etc. You can't skip these parts - and they're just as much of a challenge as temporal coherence, or performance generation or any of the other hard 'technical issues' that these models have shown no capacity to solve. They're decisions that have to be made to make a film coherent at all - not yet good or tasteful or creative or whatever.

Put another way - you'd need AGI to comprehend a script at the level of depth required to do the job of any HOD on any film. Such a thing is doubtless possible, but it's not going to be shortcut naively the way generation an image is - because it requires understanding in context, precisely what LLMs lack.