While Ren Xiaosu was still at Stronghold 109, he had heard of Fang Yujing and Cicada's names on a streetcar. At the time, he wondered if the celebrities of this stronghold were also members of the Saboteurs based on the situation with Luo Xinyu.
But later on, he felt that he was wrong to think that way. When the great disaster struck Stronghold 109, Luo Xinyu and Lu Yuan had specially waited for Yang Xiaojin in the wilderness for some time. However, no one else was in the vehicle. If these two celebrities were also members of the Saboteurs, they would have been taken by them back then.
Moreover, if Fang Yujing were a member of the Saboteurs, she would not need to approach him in such a manner. It would be just as how Luo Xinyu had greeted Ren Xiaosu openly after discovering he was her neighbor.