The Scholar's Heart was too strong, whether it was Fang Zhentian or Feng Zhe, they could only slowly refine the bit of Scholar's Heart which seeps in their defences. As for the situation outside, the two didn't have the capacity to investigate.
Thus, when Mo Wuji entered the fray and started refining the Scholar's Heart, Fang Zhentian and Feng Zhe were completely oblivious.
The moment Mo Wuji started refining, he knew that he had made the right decision. Using his Immortal Mortal Technique to refine the Scholar's Heart was like cooking a frog with warm water, no sudden actions. The Immortal Mortal Technique drew the Scholar's Heart into his circulation route, every circulation would take away a strand of the Scholar's Heart flame energy, refining it into Mo Wuji's.