Copyright infringement can happen without intending to infringe copyright.
Various music copyright cases start with "Artist X sampled some music from artist Y, thinking it was transformative and fair use". The court, in some of these cases, have found something the artist _intended_ to be transformative to in fact be copyright infringement.
> You can't copyright artistic style, only actual output
You copyright outputs, and then works that are derived from those outputs are potentially copyrighted. Stable Diffusion's outputs are clearly defined from the training set, basically by definition of what neural networks are.
It's less clear they're definitely copyright-infringing derivative works, but it's far less clearcut than how you're phrasing it.