What if, What if, What if. We can let ourselves spin and spin and spin over the what if's. The letter talks about what he wants to do, not what he's actually done. They're just words until he puts time and action behind them. And how nice of him not to blame you for your current feelings, very noble since he's the one doing damage by drinking.
IMO, don't let your guard down. The A is really good at getting us to do that. One nice letter with sweet sentiments and we're right back where we started. Keep your boundaries, keep working on you, let him work on him. If he means everything, then you will see evidence of that as time goes on. But not today. He is still drinking, nothing has changed yet.