As those men who had killed Noah returned toward their homes, they met Gideon’s men. The story is told, and Mormon abridges it severely. Mormon is providing historical details that will be important to future aspects of the story he intends to tell.
The most confusing passage is verse 24, which says “after they had ended the ceremony … they returned to the land of Nephi.” Royal Skousen has suggested that the word ceremony should be rather seen as sermon. Frankly, all explanations fall short of full understanding. We simply don’t have enough information to understand what happened.
What we can see from these verses is that something happened when the two groups who had been inhabitants of Lehi-Nephi meet and exchange information. Only after stating that they returned home do we have the statement that the king of the Lamanites swore an oath not to slay them. There is no information on why that should have happened either.
It is most probable that the information in the original record of Zeniff would have explained these events more fully. They are peripheral to Mormon’s interests, however, and it appears that he abridged out a little too much information.