This is the hall of mirrors. Where you will face yourself. Buckle up.
You know that it's difficult for people to understand all the "technicalities" of infidelity if they've never walked the path. I say cut the guy some slack. He's on the outside looking in, only knowing what his wife has said of the situation.
And let's be honest. Your ex-boyfriend has turned into a jerk. Seriously. He's a cruel, selfish, a-hole. Does it condone your cheating? Absolutely not. However, he's still a cruel, selfish, a-hole. If I was looking from the outside in, seeing my wife's BFF dealing with an abusive jerk, I too would probably think that you'd be better off with the AP. Because after all, I wouldn't know that broken attracts broken, and everything else we learn going thru the University of Infidelity.
I don't think his statement is a massive red flag that means he's going to inevitably going to cheat on his wife. I think he's (somewhat ignorantly) trying to find a safer, better solution for the situation you're in.
"Courage is being scared to death and saddling up anyway." - J. Wayne