Was this a red flag of manipulation?

Whenever we'd talk about our relationship and show appreciation for each other as a couple, my ex would always bring up his exes, one of them especially. He would draw the comparison to explain why our bond was so much better, saying that in the past he'd always found himself in toxic situations with girls who would "put up walls" and he always ended up "putting himself second" in the relationships. He would repeat this exact phrase (putting himself second) all the time. He's been in therapy before, and he always seemed to be quite selfaware, so I just thought it was based on real introspection and honest self reflection.

However, after a year together he blindsided me with a breakup blaming a month old argument (which was our only argument ever, caused by the fact that he reacted poorly to me setting a boundary, which he misunderstood as me being uncaring and not wanting to tend to his needs and feeling like he was a burden). He again explained the breakup by saying that I was putting up a wall and he didn't want to put himself second again. I thought it was weird because I've been nothing but loving and open for a whole year, and it seemed crazy. He didn't even try to have a conversation to fix it, he just made up his mind on his own, over an old argument that I thought was resolved, and even replaced me with a coworker in less than a month.

My question is, do you think that he simply misread a situation and projected his insecurities (linked to past relationships) onto me and ran away out of fear? or was he simply manipulative, and the ex discourse was triangulation to prevent me from ever setting boundaries or not being 100% available?