No, it wasn't. Theft is a well-defined word. When you steal something you take it away from them so that they don't have it any more.
It wasn't even a case of copyright violation, because no copies of any of Rutkowski's art were made. The model does not contain a copy of any of the training data (with an asterisk for the case of overfitting, which is very rare and which trainers do their best to avoid). The art it produces in Rutkowski's style is also not a copyright violation because you can't copyright a style.
There is no ethical stance for letting billion dollar tech firms hoover up all the art ever created to the try and remix it for profit.
So how about the open-source models? Or in this specific instance, the guy who made a LoRA for mimicking Rutkowski's style, since he did it free of charge and released it for anyone to use?