These verses were to explain to Israel how they would survive after the Assyrian destruction, and Nephi would have added the Babylonian destruction. Even though they were destroyed, they would yet become numerous. They would be so numerous that they would look for more land upon which to dwell. The prophecy is that the gentiles would help to restore them.
This part of Isaiah was so important that Nephi not only included it here, but told Jacob to preach a sermon on this text (found in 2 Nephi 6:6–7). Nephi was part of the families that were led to the New World, but Nephi later explains in 2 Nephi 5 that they separated from Laman and Lemuel’s families. Thus the Old World peoples among the Nephites were probably few. The best understanding of how these Old World peoples survived in a new world and in a new environment was to merge with the existing peoples. For Nephi, these were the gentiles. It will become clear in the discussion of 2 Nephi 6 that Nephi saw these gentiles as the salvation for his old world contingent.