Another case of study design not really supporting the headline. According to the paper, the comparison here was "no advice" or "AI advice": there was no "placebo" where you have advice / references from non-AI materials. Think e.g. Google's instant answer box, pre-AI. It's hard to say without the comparison, but I have to imagine displaying that would have similar effects.

There's plenty of other issues with the study, one of the primary ones being that they chose a relatively 'dumb' AI (Step 3.5 Flash), but also, specifically hand-selected wrong answers. People that are used to using competent AIs that are mostly correct would be operating off of experience that suggests they should trust AI. In this case, by design, they shouldn't have, but it's hard to fault the user for that.