Nobody's ever claimed that!
And a pilot who pulls back on the stick/yoke at high altitude is doing it wrong. Having coupled controls wouldn't have prevented him from doing so.
No, it wouldn't have. BUT if the control sticks were coupled, there would have been a tactile indication to the pilot in the left seat that the pilot in the right seat had yanked the control stick back. That would have led to the left seat pilot taking corrective action. Because there was no indication coming from the decoupled control sticks, the left seat pilot wasn't clued into the primary problem. You do understand that, right? You further understand that it is right and proper to call an engineering decision that reduces safety "a design flaw", yes? It's not the only design flaw with an Airbus, but it is a spectacularly stupid one.