The fact that the Taiwan's Civil Aeronautics Administration has ordered the pilots to undergo proficiency tests tells me they probably know exactly what happened and why.
And apparently 20% of them failed even the oral test.
Without knowing the exact nature of the oral test I don't think we can be too sure what this proves.
I would be more interested to see a pilot's performance in the simulator and/or the aircraft.
Don't get me wrong I'm not saying pilot's shouldn't be able to field relevant oral questions but there is a difference between "espoused theory" (what you
say you would do in a given situation) and "theory in use" (what you
actually do).
Whilst you would expect all pilots to know items such as Memory/Recall items etc this reaction seems like a "trapping" exercise rather than remedial/refresher training. Sort of shutting the stable door after the horse has bolted.