Jesus answers what might have been another question on the people’s minds. If Jesus had taught in the Old World, did that mean that he had told the Old World people about this people, and that perhaps there would be other Old World people (tribes) coming? That would happen in the future, according to Nephi, son of Lehi’s prophecy. Was that coming soon?
Jesus tells them that he did not let the Old World know about them. This people, and others that the Father had led out of Jerusalem, were known to God, but not to those of the Old World.
Nevertheless, even though they were currently multiple separate peoples, they would all have the chance to receive Christ’s teachings. They would all be brought into the fold of the Shepherd.
The contrast with these people who had been led away were those people in Jerusalem itself. Those people had not understood the gospel, and therefore they were not made aware of the peoples in other lands who were receiving the Gospel.