Mormon uses this scene as a description of a spontaneous conversion of Lamanites to the gospel. It makes a good story, but may not have happened in this way. For a true conversion, these Lamanites would have had to have heard the gospel, but decided against it at the time. They would also have to know that the people of Anti-Nephi-Lehi had embraced that gospel, and that the actions of those people was related to their new belief rather than any other possible explanation. All of these things would be difficult to have combined in the minds of the converted Lamanites.
Since we are getting this story of conversion from Mormon some four hundred years later, and Mormon is clearly writing this as a moral story, he may be forgiven if he perhaps improves the story with this spontaneous conversion. The certain historical information would be that certain of the Lamanites threw down their weapons and ceased to kill. What went on in their minds would be unlikely to have been reported in the original source, and is much more likely to have been Mormon’s interpretation of that action.