This type of teaching is essentially transferring the instructional scene from the Divine Arts stone to reality. Without the time constraints, as long as the instructor doesn't mind the trouble, one can keep on comprehending until they have learned it completely.
Mu You only understood one ten-thousandth of the Godly technique after one round of teaching, and that was with a successful comprehension attempt. Since each comprehension attempt could fail, one would need at least tens of thousands of attempts in reality, which shows the extreme difficulty of learning this thieving skill.
In the extremely limited time provided by a Divine Arts stone, full comprehension is very unlikely.
However, Mu You had the Pocket Watch as a foundation, so he had no such worries. With infinite reversals, in theory, no matter how difficult, he could learn any Divine Arts in one go.
