It's easy to defend against an obvious attack, but it's hard to guard against something you don't know is coming!
Since he had already decided to deal with Steve, Yang Ling wanted to obtain victory at the lowest cost possible. He did not wish to suffer a loss of eight hundred men to kill a thousand of his enemies. Steve had offered to confer a piece of territory to him, and Yang Ling decided to go with the flow, first giving him a sense of stability, then dealing the fatal blow while the Banto army's defenses were weakest.
Yang Ling believed that after Steve made such an offer, he would have to depend on Yang Ling until his reinforcements arrived. He would not suspect Yang Ling for the time being, and that was a great opportunity. As long as Yang Ling planned this carefully, Wissen Town would become Steve's burial ground.