I, as a non-statistician, agree that he was bluffing. If I were a student in that class with a high grade, cheating or no (because even the innocent may get accused and face the ostensibly severe consequences), I would be thinking "Hmm... Now how exactly does he plan on proving that I cheated?" Without a confession or third party who could reliably state that they know hypothetical-I was cheating, the possibility of actually proving misconduct on an individual level seems extremely remote.

I would love to hear some statistics folks weigh in on this. Or: speculation ftw!