Nigel, I don't see what you mean by that statement. I think it's clear by 4Greens' question he knows the differential px is high.
He's just wondering how it's allowed.... woodpecker answered that.
Now the thing here is, px breathing is great if you can get the mask on. On a sudden decompression on that altitude you have only a couple of seconds, a little more if you don't smoke. The decompression will create such a sudden huge differential px on the pilots, that it seems unlikely they will get themselves together, orientated and masks on in time.
Cessna has an auto-turn auto-descent for this very reason on their latest bizzjets.