Although the question is about the allegory of the olive tree, Nephi has just seen a vision of how that allegory plays out. Thus, even though the vision did not show the allegory, Nephi can understand it because he has seen the future history behind it.
The beginning of the allegory is the separation of the branches, and Nephi declares that Lehi’s family is a branch broken off. Just as Nephi’s vision also saw the role of the Gentiles, the allegory speaks of them. Therefore, the Gentiles will also become inheritors of God’s promises. They are not of the original tree, but will be grafted in.
The point of the allegory is to see that the seed of Lehi and the wild branch of the Gentiles all become inheritors of the covenants of the house of Israel. They will all come to the true fold of God.