Reading the journal paper this article referenced, I was a little disappointed that there weren't more variables accounted for regarding the participants and the texts. With the participants, factors I see commonly accounted for are levels of education, socioeconomic indicators (say income), or how many books the participants have read in the past month or something similar.
Regarding the text, some form of analysis to then account for or attempt to equalise the AI-generated texts so that they're similar to the author-written ones would have been nice - like average sentence lengths or average syllables per word.
However, this is still a data point (and a scary one) that it can be very hard to distinguish AI-generated content from human-generated ones..
Oof, I didn't even catch or look up the Prolific bit!