The future history of the restoration of the Gospel was given on the previous day (3 Nephi 16-17). The people therefore had a reference for the future position of the Gentiles as holders of the Gospel. Nevertheless, this was to be a temporary condition. Eventually Israel is triumphant, a point that the Savior makes here by citing Micah. (This theme is stated in 3 Nephi 16:10-13).
Reference: These verses reference Micah, who was a contemporary with Isaiah:
Micah 5:8-9
8 ¶ And the remnant of Jacob shall be among the Gentiles in the midst of many people as a lion among the beasts of the forest, as a young lion among the flocks of sheep: who, if he go through, both treadeth down, and teareth in pieces, and none can deliver.
9 Thine hand shall be lifted up upon thine adversaries, and all thine enemies shall be cut off.
The Savior had admonished the people to search Isaiah, but not Micah. Perhaps the admonition was intended for further study of Isaiah, and not related to the specific scriptural citations in this discourse. These are given from multiple sources without obvious attribution (as was practice). The people are expected to know their scriptures well enough that they recognize the cited verses without having attention being called to them.