Marking it as confidential in an email is not them telling you that you can’t share your own info. It’s warning you that the info inside the email may be something you don’t want to open when presenting etc.

A lot of companies will try to discourage you from talking about your pay. Unless they actually take action against you (i.e. teprimand/firing) there isn’t anything that you could bring to HR or a court.

Your reaction seems extreme for what would be typical corporate policy (i.e. confidential on an email like that).