Not Too Talented
Xue Wei was aware that the middle-aged ghost had realized that cultivating was possible.
He had no intention of hiding it from him. Although the ghost had been rather profit minded, he had helped them, and had not done anything to cause trouble, so it was only natural that Xue Wei also repaid the kindness somehow.
Also, the jade tablet that the middle-aged ghost had been given was already used up. It could not impart the cultivation technique into another ghost's mind.
Unless the middle-aged ghost found another way to copy the knowledge within his mind, then it would be impossible to spread the cultivation technique to others.
The jade tablets were an item of the living world. They did not exist in the Ghost Realm, and could only be taken out by Xue Wei.
The middle-aged ghost did not mind it either. Naturally he would benefit more if he was capable of selling the method, but without the Marrow-Cleansing Pills, the cultivation technique had proven to be useless.