The theory that understanding is fundamental to learning is certainly accurate, and anything the student needs to know should be taught for understanding first. However, after the student understands, they need to be able to recall, with a high degree of accuracy, when they need to apply that which they understood. To do this well, the student needs to recall what they need as instantaneously as possible. That comes by a variety of means. A high emotional impact makes things easier to remember.
Mastery is important after teaching for understanding.
Both must be taught. It is true that some students memorize very rapidly, and that others need to put in more time doing it. Both should be provided for. This, again, is not a position for or against the one who needs more time for mastery. That student may be a very outstanding producer in many ways with what they have learned and they may go on successfully to more advanced learning.