Though the Book of Mormon text uses “mine arm is lengthened out” instead of the KJV Isaiah’s “his hand is stretched out still” (such as in Isaiah 9:12), Nephi is clearly returning again to Isaiah’s literary well to draw out his imagery. The meaning is the same.
At the beginning of this chapter, Nephi has an implicit division between the promises to the house of Israel and to the gentiles, with the break occurring between 2 Nephi 28:2 and 3. While much of the text that follows from verse 3 might also be seen as a condemnation of an apostate Israel, this conclusion points it most directly at the gentiles. The Book of Mormon serves as a corrective text for both Israel and the Christian gentiles, and the most pointed portion of this chapter is directed to the Gentiles.
Just as Israel had the presence of the Lord and his protection evident with them, yet were able to deny it, so the gentiles will also have the experience of having the blessings of the Lord, but denying them in turn. It should also be noted that Nephi makes to particular distinction in his condemnation of the wicked, whether they be Jewish or Gentile. Both peoples who have had the opportunity to walk in the way of God, and choose not to, fall under his equal condemnation.