AI does give itself away over “longer” posts, and if the tool makes about an equal number of false positives to false negatives then it should even itself out in the long run. (I’d have liked more than 9K “tests” for it to average out, but even so.) If they had the edit history for the post, which they didn’t, then it’s more obvious. AI will either copy-paste the whole thing in in one go, or will generate a word at a time at a fairly constant rate. Humans will stop and think, go back and edit things, all of that.

I was asked to do some job interviews recently; the tech test had such an “animated playback”, and the difference between a human doing it legitimately and someone using AI to copy-paste the answer was surprisingly obvious. The tech test questions were nothing to do with the job role at hand and were causing us to select for the wrong candidates completely, but that’s more a problem with our HR being blindly in love with AI and “technical solutions to human problems”.

“Absolute certainty” is impossible, but balance of probabilities will do if you’re just wanting an estimate like they have here.