You're right; this study would be equally unethical without AI in the loop. At the same time, the use of AI probably allowed the authors to generate a lot more

You're right; this study would be equally unethical without AI in the loop. At the same time, the use of AI probably allowed the authors to generate a lot more comments than they would have been able to manually, and allowed them to psychologically distance themselves from the generated content. (Or, to put it another way: if they'd had to write these comments themselves, they might have stopped sooner, either because they got tired, or because they realized just how gross what they were doing was.)