He is not saying Michael to STFU, he is telling him that if he had accused his manager of lying in the face, he would have been shown what the scores were just so that he would 'STFU'.
It seems entirely unambiguous that he's saying "You can get your manager to show you your scores by making enough of a fuss that your manager will do it just to make you STFU".
The manager may have wanted to do that, although that is purely conjectures, but the manager didn't, perhaps because it's against company policy, but also perhaps because his manager didn't want to go all out on Michael, perhaps realizing that the faults in the matter might not have all been on Michael's side.
So, then, I think Googler123 is telling Michael to STFU not by telling him directly, but by putting words in his manager's mouth, which is I think even worse, because not only is this denigrating to Michael, but also to his former manager, who may still be a Google employee.
Makes sense?