It's impossible for us to work out how well Mann defends on his player stats because we don't have a breakdown on how he goes in different positions, and he has played in all sorts of positions.
His stats would look great if you thought of him as a winger or fullback, but terrible if you thought of him as a 5/8.
O'Brien seems to think he is a good defender, and frankly, O'Brien is one of the few people who has spent years working with Mann and looking closely at his skills and at his stats in various positions (as a coach at Melbourne), so can make an informed call on Mann as a defender.
I wasn't impressed with how he went at 5/8 last year (in defence), but clearly he was injured, being targeted, and was not getting much help from the players around him, many of whom were not really having a go by the time Mann was dropped into 5/8.
In short, I think O'Brien is in a much better position to make this call than anyone else, and if he says Mann is a good defender, and he does say that, I'll wait till I've seen different before making up my mind.