These young men’s mothers obviously possessed tremendous faith. All of those women were survivors of a slaughter launched against them and their kinfolk in the land of Nephi. They had faced what looked like certain death for them and their children—possibly some of these same boys had been babies or toddlers at the time—and had faith sufficient to keep their oath. That incredibly strong faith was passed on to their sons—not that obedience would mean salvation in the next life, but in this life: that Yahweh would deliver them if they were faithful.
This teaching is so similar to the Nephite foundational promise that we may read it as part of that promise. Yahweh had promised the people of Nephi (2 Ne. 1:19) that, if they were righteous, they would be preserved in the land. The Nephites adopted this covenant as the one of the fundamental underpinnings of their collective social consciousness. These Ammonite mothers obviously took the promise literally and taught it to their sons. If Yahweh had said that his people would be preserved upon condition of righteousness, then their sons would be, too.