They said earlier on that the sutra itself included both a cultivation technique and a fighting style, so probably only the cultivation technique can increase willpower.
Even though the benefits of the Seven Wounds Demonic Sutra were great, they could not be used often. Moreover, the Seven Wounds Demonic Sutra could not increase one's mental willpower. Only by practicing the Seven Wounds Demonic Sutra could one increase one's mental willpower.