Isaiah had spoken of a gathering of Israel. Nephi sees that as occurring in the end times, sometime after the Book of Mormon had been published. The previous verse declared that as Nephi’s seed turned to Jehovah, they would become a pure and delightsome people. In verse 7 Nephi says the same of the Jews. After they are gathered, they shall also believe in the atoning Messiah, and they too would then become a delightsome people. The use of delightsome is not to suggest that they were loathsome before, but only that there is a contrast of opposites. In the final days, there is a division into those who follow God and those who follow the adversary. It is in the context of that contrast that we should understand how the Nephite seed and the Jewish seed would become delightsome.
As Nephi describes the gospel being taught to all peoples, he uses that event to tie his prophecy with Isaiah’s. Verse 9 quotes, with a minor introductory change, Isaiah 11:4, (also 2 Nephi 21:4).