Buneng left after receiving the answer he wanted to hear. Before departing, he told Shen Lian that he would deliver the Mahayana Sutra the following day. He did not say what would happen if Shen Lian's suggestion did not work out.
This was because Shen Lian just did them a big favor by giving them his suggestion.
Furthermore, the Sutra was not a living being. If it was someone else other than the Buddha or the Heavenly King who obtained it, the person would probably not be as accomplished as Buddha and the Heavenly King.
The Mahayana Sutra did not help much in the Dajue Temple. Otherwise, they need not to be fearful of Emperor Xia at all.
After Buneng left, Shen Lian took out the black metal token which was the personal order Emperor Xia delivered the night before. The token did not only contain Emperor Xia's invitation to conquer the Eastern Barbarians, but also his determination.