The point isn't about the perceived quality of AI-generated artwork but the fact that the tools like Midjourney and whatnot have been built on databases of unethically-sourced and uncredited artwork (or at the very least, being totally opaque about where it all comes from) of many human artists, not to mention the infrastructure that powers them being particularly ecologically-unfriendly.
Authors disclaiming the usage of such generative artwork does not change that fact, and it's frustrating to see it being tolerated and normalized like that.