Gu Xiaonuan took advantage of Mu Shan's complete abandonment of trying to persuade her and straightforwardly told Mu Shan what she truly thought.
Including that she already had someone she liked, which she had also told Mu Shan without reservation.
Mu Shan knew that Gu Xiaonuan already had someone she liked, but that person wasn't Si Yuchen. What else could she say?
"I still feel it's a pity, but there's nothing to be done. If you don't have feelings for Chen, then being forced together won't do any good."
Mu Shan hopes that Si Yuchen can find happiness, but not through such a forced situation.
Gu Xiaonuan's lack of affection for Si Yuchen was something Mu Shan had anticipated.
Otherwise, Gu Xiaonuan wouldn't have tried everything to divorce Si Yuchen, which was enough to show she still didn't have feelings for him and didn't understand his character.
As someone who understands Si Yuchen the best, Mu Shan could see clearly Si Yuchen's attitude and feelings toward Gu Xiaonuan.