Doesn't really matter, The way I see it is Back of head strikes are banned mainly in usage as a strategy, particularly if the opponent is in a vulnerable position defending a rear naked choke or something like that....
With The angle and distance that matt brown throws the elbow from, as well as the way diego is holding himself, head bent downwards, it's not really obvious he was aiming for the back of the head, which is key....
also with elbows, forearms, shins etc there is a long surface area to possibly contact with the back of the opponents head....it doesn't mean however that the strike was irresponsibly thrown or shouldn't have been thrown, it's just the luck of the sport