This passage is fascinating, both for its biblical content and for the fact that Fray Durán recognized and discounted it as a story from the Bible. More interesting, however, is the fact that a native informant gave this material in response to a question about Quetzalcoatl. I propose the following speculative reconstruction of his thought processes.
The native informant obviously had learned the story, either directly or at second hand, and could relate it with enough details that Durán easily recognized it. Second, the native had to find enough correspondences with the Quetzalcoatl material as he understood it that he could combine the two stories. In the reconstructible native legend, Quetzalcoatl got to the seashore and departs from his people. In some versions, he is accompanied by a number of followers, and miracles occur both en route and on the beach. Quetzalcoatl also has a symbolic association with serpents. (See “Excursus: Quetzalcoatl: A Malleable Mythology,” following 3 Nephi 11.) These points are conceptual nodes of meaning that parallel the story of Moses. Thus, the native had to know both tales, find the similarities, and use them to reconstruct his own version of the Quetzalcoatl material.
What we see in Durán’s experience with his native informant is the same process I suggest for Mosiah2’s translation. The material being translated and Mosiah’s understanding of the scripture had enough resemblances that Mosiah shaped the Jaredites’ original story to match the brass plates’ story at a crucial point.