I don’t really find it that surprising - if you write generic low-quality stories, it’s difficult to differentiate your work from an AI writing generic low-quality stories. People have been selling slop stories on Amazon for a long time before AI tools, so describing them as professional authors is not exactly damning here.
Okay, but if your argument is that people couldn't differentiate them because these particular authors also write "slop" then you should just lead with that. I think many people would reasonably disagree with you.