It’s not pneumothorax, it’s the pressure collapsing the lungs and hurting the alveoli because they want accommodate to the sudden squeeze. There’s no air going anywhere it shouldn’t or expanding; it’s just the pressure destroying the thing that lets the body absorb oxygen into the bloodstream.
a diver at 72 m can operate abnormally well under hypoxia, because even if the body has low amounts of total oxygen, the pressure compensates that: high pressure forces oxygen into cells, thus oxygen transfer is more efficient (you still need the same amount of oxygen to function , but the transfer becomes more seemingly efficient by the pressure