Oh @TonyBravo I say again a fucktundrum - his past little lies probably happened years ago (thinking Covid and trains) but clearly left a mark that has clearly started to undermine your ability to trust!
Regardless what and who your privacy has been invaded. Everyone is so focused on your suspicion being correct, so I'm going to just say some counter points, but before that you said battered years later?! Is DH emotionally abusive??? If so jump out of that!!
Just a curiosity, was it that DH brought up all these ancient posts that had nothing to do with you? or that he read something and then took years to confront you with it?
So, as the tiniest of points, the fact that he got the wrong "you" means that he was searching, and not in your stuff, its still fucked up but DH not going through your messages. Do you know what his intention was?
Then it comes down to the invasion of privacy, and any which way its still awful regardless of it. I say it again, fucktundrum! If he's tried to tell you that your privacy was breached would you have rather DH didnt say anything?
Do you think that with the immediate jumping back to things in the past and not necessarily recent, the messages DH had seen or heard was something to be defensive over even if there isn't anything to hide?
This is a total lose lose situation, either he's done the right thing and its shown that there is something confrontational and broken between you, or he's done the wrong thing and the way you describe him a manipulative and/or controlling twat that deserves whatevers coming.