Really, that rule is interpreted as part of the revealing of private information rule. Basically, only if it contains sensitive information or the person does not consent to have it aired is it against the rules. However, much like in an actual lawful society, once you break the law, you forfeit other rights. His PM to me was a blatant attack, and hence negated his right to privacy.
Similarly, if one of you were to post a conversation with someone where they were being a blatant asshat towards you, I'd have no qualms with it so long as it didn't reveal any identifying information/real personal issues. The rule is in place to protect private information that the user would like to keep private. In this case, daniel99 essentially broke the flaming + defamatory rules and hence forfeited his right to keeping that PM private.