Cheng Shuyu only confirmed the news when she overheard Di Jingkun's conversation with his assistant. She realized that her daughter was now being raised as a young miss of a wealthy family. Then, she learnt that the girl that was inside her house was the child of that wealthy family. That's when everything changed.
The hatred that Di Jingxuan and his wife had towards Di Zhong was not something that could be dealt with by just a simple apology or by giving Di Jingxuan the throne. Even destroying the royal family would not be able to compensate for what had been done to their family.
Di Zhong was the one who caused Di Jingxuan to lose both his parents and the reason why his children were separated from him.