This response is basically culture war bickering.
The parent didn't imply that slop didn't exist pre-AI, so that's a purposeful mischaracterization.
The simple fact is that AI absolutely exploded the amount of slop and, at the same time, lends itself to concealing the source of slop. There are other variables too distinguishing the human reaction to AI slop from the human reaction to human slop, but even ignoring that: Just the two things I mentioned, together, mean that, yes, of course, any rational person is going to constantly question (internally or out loud) whether things are genAI. So if "rational" is "good", then yes, the backlash is good.
It's unreasonable to require evidence before even saying that something seems like bad genAI. And it's only a "prejudice" or "bad attitude" if you disagree with the premise (that genAI content is often bad in some way, either intrinsically or practically). Which, fine, make that argument. But that's not the argument you're making.