I would take a relationship with a person working a good committed AA programme over your average bloke anyday.
People can be become not only better versions of themselves through active recovery but better versions of themselves than they’d have been if they didn’t have the guiding principles of AA.
AA teaches honesty, accountability and acceptance. People in AA still fuck up at life without it being relapses of addiction. Just being a selfish dickhead or causing an argument and being pigheaded etc and they almost always OWN IT fairly quickly, reflect deeply on how they’ve behaved, make amends and work a bit harder on themselves. They have a whole squad around them setting them straight, calling out dickish behaviour not just agreeing and fluffing their ego.
However, not every in AA is well, or is working a proper honest programme. These people are unpredictable and dangerous (and often highly manipulative, weaponising the fact they are in recovery to avoid accountability at home/in outside life, as if just getting their arse on a chair for a few hours a week = changed person. It doesn’t)
It could be possible someone with issues in AA has taken against your partner. It happens when people are still very ill and full of twisted ego. Your partner will likely have shared things about his past and current life thinking that’s a safe space. This person could have taken these bits of info, the fact he wasn’t a good partner to his exs etc, then used his social media to work out who you are and grabbed a big old shit stirring stick and messaged you something inflammatory. Like I say not all who are in AA are well and some are still very sick and absolute shit shows of behaviours, even without a drink in their hand.
I’d ask him to think if there’s anyone in the rooms with him who feels off, or seems to not like him, or he’s pulled up on shit behaviour and might have left someone wanting to take him down a peg or two. Because I honestly don’t think it’s an unimaginable situation, that’s where this bullshit is coming from.