Of course Jiang Yu was telling the truth.
Li Yue also knew that Jiang Yu was telling the truth. But she was just unwilling to accept it. Moreover, she already knew grandfather Mo's attitude towards Jiang Yu -- he had already acknowledged Jiang Yu's identity as his granddaughter-in-law.
Grandfather Mo had always advocated for Mo Long to marry her, so she relied on grandfather Mo's special preference for her. Li Yue thought that even if she delayed the wedding date, as long as Mo Long's leg was cured, then she could marry him immediately.
But she never expected that Jiang Yu would appear. She cured Mo Long's leg and obtained Mo Long's heart. She originally thought that Mo Long's interest in Jiang Yu was just a momentary novelty. Anyway, in the end, the person Mo Long would marry would only be her, Li Yue.