Mo Wuji stared at Bai Ye and even though he didn't possess the Dao of Prophecy, Mo Wuji suspected that Bai Ye had a hidden motive.
It was true that Bai Ye had great enmity with the Great Sword Path but Mo Wuji found it suspicious that Bai Ye would wait for them to come over before joining hands against the Great Sword Path.
Mo Wuji knew how cautious and scheming Bai Ye was so if he knew that Ping Fan Immortal School was going to fight the Great Sword Path for him, he definitely wouldn't join the force. He would at most watch the show by the side and act only against those few loose fish which escaped the net.
Moreover, he would only need a while more before being able to advance to become an advanced Immortal Emperor. Even if he wanted to be part of this revenge mission, he wouldn't have done it while he had yet to advance into the advanced Immortal Emperor Stage.
Therefore, Bai Ye sudden inclusion did seem slightly odd.