It's not balance because the team isn't centred around a defensive backbone. It hasn't been made because of the defensive synergy but rather you have a number of mons who rely on offensive pressure and momentum in order to win games.
With balance, a large part of the game is being able to switch out into bulkier mons who can heal off the damage, whilst with BO, in general doing this kills the momentum of the team and due to the frailer nature of the team, allowing your opponent to bring in a breaker means you're in a weaker position.
A great example would be looking at Mega Venusaur; whose defensive sets only really fit on balance, due to being forced to recover so option. On balance, you can bring it in on a mon, then after your opponent goes to Mega Pinsir (for example) you could go to a defensive answer such as Skarmory. But on BO, chances are, you won't have a mon who full on counters such a defensive mon and would therefore have to sack. Instead you would have been better trying to predict Pinsir coming in and doubling out to an answer such as Tapu Koko.
Of course, BO has defensive switch ins to some mons, or else it'd be Hyper offence. Mons found in this group are mons like Defensive Lando-T, Tangrowth, Mega-Scizor, Rotom-Wash, Ferrothorn. But they're more pivots than walls, helping to gain offensive pressure by VoltTurning/Setting hazards/Threatening with coverage or status.
TL/DR: Balance: defensive synergy + a small offensive core
Bulky Offense: offensive synergy + a small defensive backbone.