The sudden blast of Yin-Evil Ice Palm from Duan Tianya was clearly a demon tao technique, not something he learnt from the Shaolin Temple. If the situation hadn't been so perilous, he would not have resorted to showing his hand.
Demon Tao martial arts techniques have always been despised by the martial arts world. Even though Duan Tianya was no longer a disciple of the Shaolin Temple, if he used this demon tao technique in front of everyone, the Shaolin Temple would certainly not let him off.
Zhu Zefang was on the brink of death and he didn't know if he himself could escape. He had nothing to lose, so he revealed his secret weapon. Moreover, as long as Su Xin was killed, he had no fear that his friend Zhu Zefang would reveal the truth about his demonic techniques.
Su Xin turned around and launched a punch. The Vaitou Descent technique cracked the void, rocketing Duan Tianya off his feet.
Despite his injuries, Duan Tianya continued to fight and Su Xin was also in a bad way.