It was okay if they tricked him as he would have accepted any challenge as a form of entertainment. However, they had crossed the line when they got Gu Xijiu into trouble! He would let them know what the price for challenging him was, but he would put his revenge aside and look for Gu Xijiu's soul first.
Ye Hongfeng was currently possessing Gu Xijiu's body, which meant that she was also now being supervised by the Omen. Therefore, she must have a unique way to contact the Omen to report her progress after the possession plan had succeeded.
Di Fuyi did not want to alert them before he had found Gu Xijiu's soul. Therefore, he acted as usual even though he had realized that the soul in Gu Xijiu's body was, in fact, Ye Hongfeng. He pretended not to know so that he could figure out his next step.