Even if the subject is not for OP there's a plethora of better phrasing options available that are less demeaning that what the teacher used. Even if he was planning on saying it after closing the call the fact remains that he did not. I'd say the true idiot is the one who talks shit behind someone's back without making sure the person is not on a fucking call with him.

Obviously OPs thesis supervisor could have been nicer, and direct, with what they said, but he may have said it intentionally in frustration. Without a follow-up, it's hard to determine if it was accidental, or on purpose.