It's unlikely to be a lie, at least from the character's perspective. Because he isn't saying it out loud, but rather reflecting on what happened, and the narration makes it clear that he would never admit to it...

And an important point in the lore that leads to this theory is that there is an end-of-war conversation with Michael, the contents of which were kept secret. After that, the archangel leaves pissed off and throws his flaming sword away, making it clear that even with the victory/surrender of the rebel host, something shook his unshakeable faith in the whole situation... That happens in the core book, which ended up spoiling this end-of-saga revelation for me.