Nephi declares that what Jehovah said to the Jews in the Old World would be written down. He has already seen that it will be preserved. He declares that Jehovah will speak to Nephi’s people, and it will be preserved. He further declares that other scattered tribes will write. What he also declares is that the Bible and the Book of Mormon would be had in the future, but he does not make the same promise for the writings of the others, at least at this point.
What is important is that they will come together -- together to be a stronger witness of Jehovah’s word to all peoples.