Cao Cao didn't care whether Yuan Shu was destined by the Mandate of Heaven or not. Since Xun Yu's established strategy had already locked on Wancheng, Cao Cao would definitely attack Wancheng!
It was precisely because of this that Yuan Shu, who was currently in Xiangyang, hadn't yet attacked Cao Cao. Cao Cao had already captured Wancheng before Yuan Shu could react. People like Xun Yu were not the kind who would attack a city only as a last resort, especially since Wancheng of Nanyang happened to belong to the World Association at that time, with merchants from both the Xun Family and the Chen Family present there.
Before Yuan Shu's defeat at Jiangling, how Chen Xi attacked Pingyu had already been scrutinized by Xun Yu and others through their recruitment of tomb robbers. Therefore, at that time, Xun Yu arranged for people to start digging wells in Wancheng.