This is an interesting detail that certainly comes from Ether’s record. Even allowing Moroni a fairly free hand in modeling the events of history into a form that highlights the promise of the land, this incident is sufficiently different from the rest of the incidents of rebellion to tell us that all of the rebellion stories are part of the original record. Those that are similar have a relationship to this in that they are sons rebelling against fathers. That process was obviously rampant in the culture. However, this resolution by pure happenstance tells us that the stories were written as they happened, and are not simply the result of a cyclical historian.
When Shez the younger becomes rebellious, enough is known of the rebellion that it gets written down in official records. However, the end of Shez the younger is at the hand of a robber. In this case, the robber should be seen as a thief, not a Gadianton-style robber. Even though there are secret combinations, and even though Mormon’s use of robber was to be equivalent to the Gadiantons, and therefore secret combinations, the circumstances of the death of Shez the younger suggests much less organization.