Verses 1 and 4 form a repetitive resumption. The middle two verses speak of the relative sizes of the political entities. When Mormon calls the Nephites descendants of Nephi he was using that term in its generic meaning, not a lineal designation. We also learn that the people of Zarahemla were descendants of Mulek. This is the first time we learn that. We do not learn that Mulek was a son of Zedekiah until Helaman 8:21.
Concerning the relative sizes of the political entities, there were Nephites who merged with the more populous people of Zarahemla. Combined they were not half as many as the Lamanites. This relative size continues to reinforce the idea that the term Lamanite simply meant “not us” and that they represented all of the surrounding peoples, whether they had a lineal connection to Laman or not.
As Mormon moves from his aside on population sizes back to the story, he adds that as Mosiah gathered the people, they gathered in two bodies. Without stating it, it is clear that the two bodies were Nephites and Zarahemlaites. They had merged, but it was an uneasy merger.