> Since you cannot (it is technically not possible) create an ML model reproducing a style without having it ingest original works in that style

I very much disagree with this. Certainly no gen-AI model currently does this, but you could reinforcement-learn your way to Ghibli style. In language modelling, that's the entire point of the RLHF step.

As long as the style can be described or even recognized by humans, you can nudge your way towards it with multiple-choice sampling.

> such a tool is producing derivative works based on original work (let’s say, by Miyazaki or Studio Ghibli) at scale and for profit, then I have difficulty seeing how copyright law is not involved.

How is this different than a fairground caricaturist that draws subjects in the Simpsons style? They "ingested" lots of Groening-style media, and now they're creating works that are derivative of that style, at scale and for profit.

Learning from a work and being inspired by a work do not create 'derivative works' for the purposes of copyright. The boundary between that and an infringing quotation is fuzzy (note 'sampling' cases in music), but a style itself needs to be on the non-infringing side of things if art is to progress at all, even by humans.

Note that I'm leaving aside the issue of data-gathering; if OpenAI was downloading pirated copies of the Ghibli corpus then that act itself was copyright infringement, independently of the AI training and output.