The vocabulary of “the land northward” and of “the narrow neck of land” suggests that this is Moroni’s interjection. The description of the land northward being covered with inhabitants recalls Helaman 3:8 where Mormon had indicated that those in the land northward “began to cover the face of the whole earth.” When Moroni adds that they “did buy and sell and traffic one with another, that they might get gain,” we see a reflection of 4 Nephi 1:46: “And it came to pass that the robbers of Gadianton did spread over all the face of the land; and there were none that were righteous save it were the disciples of Jesus. And gold and silver did they lay up in store in abundance, and did traffic in all manner of traffic.”
Mormon had used the Jaredites as the model for the Gadiantons, and Moroni is making certain to point out those aspects of the Jaredite record that support his father’s use of the document.