Probability is not an acceptable way to determine a student's future. They may have learned from the AI and remember some of the exact phrasing, and learned writing/language cues from it as well.

Agreed. I'm not a good writer, tending to stick to a somewhat abrupt, point-making structure almost better suited for bullet pointing. I've taken tips from other HN users on how to improve, but I have no doubt that had I been going through university these days, I'd probably be flagged too.