Nephi previously noted that there was a future scattering, but now reminds his readers, through his lecture to his brothers, that there has already been a scattering. Isaiah lived at the time of the loss of the ten tribes, so Nephi declares that Isaiah’s words apply prophetically not only to his own time, but to a future. Nephi silently includes the Nephites as one of the scattered peoples.
Nephi’s point with this part of the discussion is to show that although a scattering has already taken place, even though there is another prophesied for the future, that the promises of a gathering do not apply only to the future scattered people. Jehovah’s promise is to all of his scattered children, regardless of when or why they were, or will be scattered. They will all be gathered.
Nephi also mentions that those who will be scattered will harden their hearts against Jehovah. This begins the point where Nephi’s exegesis becomes intertwined with the vision he saw of the future of God’s people. He saw this scattering and hardening of hearts, and therefore his understanding of the vision is reflected in the way he reads Isaiah.