The reason for discussing the origin of the Bible was to respond to the future rejection of the Book of Mormon with the argument that the Bible was sufficient. Nephi returns to the theme of the marvelous work and a wonder by noting that there are more nations than just Israel, and that there are Jews upon the isles of the sea. It appears that for Nephi, it is obvious that Jehovah would provide His word to his scattered covenant people, regardless of where they were. Therefore, Jehovah’s word would come to those scattered on the isles of the sea. Nephi understands that his people are encompassed in that definition.
The argument is about whether there should be more of God’s word. Nephi declares that since there are more of the covenant people, that there ought to be more of God’s word. Perhaps Nephi understood that the collected scriptures of the northern kingdom included prophets not known in the southern kingdom, such as Zenos and Zenock. If Jehovah gave His word to separated Jews who were that close, how much more to give His word to those on the isles of the sea?
Nephi declares that the testimonies of the separate peoples will serve as a confirmation of the overall word of God. That is according to the law of testimony, that it should be established by two or more witnesses.