Yes,you are right, the test can't tell you 100 percently that you dont have the skills.
but what do you think, who has better chances to pass the training. someone who had to train for it and didn't have the self confidence to do it without training or someone who is spontaneous,tried it and passed it?
you have to bear in mind, in reality, you don't know what is going to happen so you have to act spontaneously and you have to make the right decisions SPONTANEOUSLY!
and that is the reason why they don't want you to practice for it(some things such as english,describing things etc are okay but not the exact tasks!)
and dont think they will not see if you have practiced for it. the chances are high that they kick you because of that. or they raise the bar. because how could they compare you with someone else?
and the example with intelligence is stupid (in my opinion.)
do you really think you are better at multitasking or something else then someone who is not as intelligent as you?
there are some specific skills which you have or don't have. if not,then you are not able to become an air traffic controller to work with such a high workload.