Currently, Fei's dantian was filled with true energy and fire force. Although he needs fire force for his current attacks, it doesn't mean true energy is completely useless.
Cultivation is about refining energy to gain essence. And essence represents vitality, though vitality would be already merged with him during cultivation, this doesn't hide the fact about energy. After all, energy is the source of vitality.
Of course, only wood energy has the power to heal one's energy completely, but he doesn't have wood attribute in his spiritual root. So he needs to use Rapid Regeneration which consumes true energy to heal. So he needs true energy for many uses. Of course, he still has secondary storage for energy except for dantian which can only be used in emergencies.
