There's an obvious trend there of older faces being judged as human and younger faces being judged as AI. A little less obvious, it also looks like "scruffy" male faces may be judged as human while clean shaven males or males with well trimmed beards are judged as AI. I wonder if the study noticed / addressed that at all.
This is my number one giveaway for AI gen. There's definitely a style out there which only shows flat, even lighting, with the subject viewed from directly in front. AI Female 44 in the OP is one such example
If you're looking in the library for books that are at least 100 years old, you're generally only going to see the ones that people thought were worth preserving for 100 years.
If you're training your image generation model with stock photographs, you're generally only going to be giving it images of people who are literally models. Not all models are beautiful, but they're probably more beautiful on average than the general population.