Temperatures (SATs) of less than -70C are, as you imply, a bit unusual at temperate latitudes. They are common in the tropics, where the tropopause is so much higher. You have to climb high to find them, though, well above the certificated ceiling of the A320 (F/L 391?). That ceiling is, I think, based on restricting the cabin altitude to a max of 8000 ft at max-diff pressure.
But I see know reason why Airbus could not have flown high enough during certification in 1987.