There's a similar issue in chess with cheating detection. They use statistical analysis to see if someone's moves are too good. Computers play at a much higher level than humans and you can measure how "accurate" a move is.

It doesn't mean much for a few moves or even 1 or 2 games but with more data you get more confidence that someone is cheating or not cheating.

Chess.com released a rather infamous report last year about a high profile chess player that was cheating on their site. They never directly said "he is cheating" but simply stated "his games triggered our anti-cheating algorithms"

One is debatable, the other is a simple fact. The truth is an absolute defense to defamation. Hans attempted to sue Chess.com for defamation and from what I understand, the case got recently dismissed.

I'd imagine these AI detectors for schools have similar wordings to avoid legal risk. "High probability for AI" instead of saying "AI written". In that case, you may have very little case for defamation.

However, I'm not a lawyer. I'm just guessing these companies that offer this analysis to colleges have lawyers and have spent time shielding the company from legal liability.