Guess it's possible to learn anything - provided you have enough time. However that is not the case with air traffic controller training. You have a limited amount of time and that is why they need to know if you are able to learn more or less complicated things considerably fast. It is better not to practice for the FEAST test because if you do not have the ability to learn new things quickly, you are not going to make it through the training. They are not looking for perfect controllers but people who can be trained to become one.
One more thing: an unsuccessful trainee asked reason for their termination: "You are progressing, but not fast enough."