How would pure ai ever be "much stronger" in this scenario?

That doesn't make any sense to me whatsoever, it can only be "equally strong", making the approach non-viable because they're not providing any value... But the only way for the human in the loop to add an actual demerit, you'd have to include time taken for each move into the final score, which isn't normal in chess.

But I'm not knowledgeable on the topic, I'm just expressing my surprise and inability to contextualize this claim with my minor experience of the game

If you had a setup where the computer just did its thing and never waited for the human to provide input but the human still had an unused button they could press to get a chance to say something that might technically count as "centaur", but that isn't really what people mean by the term. It's the delay in waiting for human input that's the big disadvantage centaur setups have when the human isn't really providing any value these days.