The earlier version of the story of the lost expedition is given in Mosiah 8:9–13. The important overlap is the question of translation. There was a record that must have information, but no one could read it. In Mosiah 8:11–12 Limhi twice asks if Ammon knows of anyone who can translate it. In both retellings, Ammon declares that there is such a one. King Mosiah had the means wherewith he could translate.
The flashback history is complete. Mormon has caught up to the story with which he began this flashback. Now, Mormon must move the story forward. As a transition, Mormon declares the communal sorrow for their sins, and importantly, for the communal sin that led to Abinadi’s death.