It's not their work unless it's identical, but in practice generated images are substantially different. Drawing in the style of is not copying, it's creative and it also depends on the "dialogue" with the prompter to get to the right image. The artist names added to the prompts act more like landmarks in the latent space, they are a useful shortcut to specifying the style.

If you look at the data itself it's ridiculous - the dataset is 2.3 billion images and the model 4.6 GB, that means it keeps a 2 byte summary from each work it "copies".