Lin Mu's ring had managed to open a rift in the barrier of the Sacred Land. This gave him direct access to the Lesser void outside and also allowed him to see all that existed beyond it.
Originally, Xukong only intended for Lin Mu to bask in the sensations of the Lesser void and improve his understanding of the Spatial element. Till before, Xukong knew that Lin Mu needed to comprehend what he had on his own.
Thus, even if he had gotten him to check the void, it would not be useful as his perception would simply not be on that level. It was the same as giving an elementary school student the work of a college student.
They might be able to read the content, but they would probably not understand anything. It was the same for Lin Mu. Until he learned and digested what he had already been taught, it would not be beneficial for him.
But now, it was the perfect time.