One example of an issue raised was that the study completely ignored the fact that most grades are set purely by exam results, not by in-person graders, and that the exams themselves are anonymized.

This could indicate that more attractive women can get better help from their professors, classmates or various other resources it doesn't necessarily mean that their exams are being marked more favourably because of their attractiveness. Note I haven't read the paper to see if the author mentioned that as a possibility or did anything to determine that possibility, if they didn't I would guess that is likely the reason for why this result exists, it isn't necessarily professors creeping on young attractive women it could just be an unconcious bias to spend more time answering their questions and ensuring they understand, which if that's the case it's important to know so we as a society can work on better addressing these unconcious biases.