The people who were destroyed obviously refers to the Jaredites. What is not said is that the 24 plates of gold which Mosiah translates include a vision of the brother of Jared which is not revealed to the people at this time. It is not revealed because the doctrine in it is too sacred. Some of this material was not to come forth until after the resurrection of the Savior (Ether 3:21), and some of it was not to come forth until the Gentiles…repent of their iniquity, and become clean before the Lord. And…exercise faith in me…even as the brother of Jared did (Ether 4:7-8). The latter section still has not been revealed and is part of the sealed portion of the Book of Mormon.
Joseph Fielding Smith
"In translating this record Mosiah kept from going forth to the people that particular part forbidden of the Lord to be revealed until after he was lifted up upon the cross. These sacred revelations given to the brother of Jared were kept from the Nephite people, as well as many other things, until after the resurrection of Christ. After the appearing of the Savior to the Nephites, the vision of the brother of Jared was revealed to the Nephites. When Moroni made his abridgment of the record of Ether, he copied on his record the vision of the brother of Jared. At the command of the Lord, however, Moroni also sealed up the greater things in this vision and the
‘interpreters’--which were the same ‘two stones’ had by the brother of Jared, so that this vision should not be made known even in our day among the Gentiles, in the day of their wickedness; ’ … until the day that they shall repent of their iniquity, and become clean before the Lord.’ (Ether 4:7) So we today do not have the fulness of the account written and sealed up by the brother of Jared and again sealed by Moroni. This part of the record the Prophet Joseph Smith was forbidden to translate. We have, then, received but the ’lesser part.’" (Answers to Gospel Questions, vol 1, p. 162)