They take away is contained in the first and last sentences.
Derivation is key.
Copyright protects original expressions, and copying means to reproduce (read and write) something. The analogy OP made is focused on the reading and writing done by humans and the reading and writing done by an algorithm.
Algos like stable diffusion are reading, and their user is controlling what is written.
If the user produces a work that is unique, but uses the style of a particular artist, that seems like it should be valid, since style is simply a process. It's how to create art, but it is not art, and processes are not subject to copyright based on copyright.gov.
With all that said legality isn't morality, and I sympathize with the artist.