No, for given the great mysteries revealed in the parables, their Yes
in Matthew 13:51 could not show complete understanding, but they did have a grasp of the essential things—a knowledge that sets them apart from the crowd (see Matthew 13:14, Matthew 13:34). In a sense, Jesus here is putting the apostles to the test, giving them a recognition of their true spiritual state.
No, for in light of the nature of the mysteries revealed in the parables, their affirmation in Matthew 13:51 can hardly be understood as a complete comprehension. Rather, it refers to an understanding of the essential things—a discernment that sets the disciples apart from the crowd (see Matthew 13:14, Matthew 13:34). In a sense, Jesus here in Matthew 13:51 puts them to the test, leading the apostles to a recognition of their true spiritual condition.1
51 “Have you understood all these things?” They said to him, “Yes.”