The problem is Dozy a level bust will get you stood down which is why the pilots probably reacted the way they did. An overspeed with both pilots applying a sidestick input will achieve the same result in terms of CC injury.
What is interesting is this is a classic HF incident. The PM reads back the correct assigned level but then hears the PF say "FL 240 blue" and acknowledges that as "240 checked" The PM then plays it all back in his mind until he realises there is something not quite right. The PF has only heard "FL240" in all the transmissions and readbacks. If there not been any discussion about FL200 by LIPGO by either the crew or ATC it possibly would not have happened.