Ctrl+F for the heading "What if AI becomes capable of writing truly reliable Wikipedia articles?" in the article - it discusses that it was started because AI-written content is low-reliability, but that could change as AI improves.

It's a pretty short article compared to what usually gets posted here.

Well, I wasn't very active between February and August, but from what I understand the guideline was only changed in March to disallow AI edits entirely instead of allowing them if you pinky promised you've checked your sources and they really do say what you're putting into the article because a lot of people were pinky promising and those promises turned out to be either lies or bullshit when people actually looked at it.

So the answer to "Why do we assume AI-generated means slop?" is probably "experience", and any proposition starting with "If the content is good enough" is trivially true as a conditional statement with a false antecedent (i.e., ⊨(⊥⇒Q) for all Q).