I am also baffled that people were so brazen as to openly cheat in a group of 70+ people. Did they honestly believe that 70 people would all keep your secret and not try and get you failed out of spite? Like, I don't condone cheating but that's just shit ops.
Further evidence that they didn't necessarily even realize they were cheating - aside from the exams - and that the professor should have spoken up sooner instead of playing undercover cop.