The context of verse 30 is that the Americans would not utterly destroy the native American Indians, described by Nephi as the mixture of thy seed, which are among thy brethren. But how could Nephi's seed be alive in the 18th & 19th centuries? The simplified version of history records that the Lamanites killed the Nephites in about 385 AD. However, the Book of Mormon record states that there were Lamanites among the Nephites and Nephites among the Lamanites. These had chosen their allegience based on religious and political lines and not racial lines. Therefore, it should not be surprising that the promise was given to Nephi that some of his seed would be preserved even after the final destruction of the Nephites. This means that some Nephites who had defected to the Lamanite side would merge with Lamanite society (see Alma 45:13) and the blood of Nephi would be preserved.
Mormon records that some Nephites had been mixed with the Lamanites, And they (the records) have been handed down from one generation to another by the Nephites, even until they have fallen into transgression and have been murdered plundered, and hunted, and driven forth, and slain, and scattered upon the face of the earth, and mixed with the Lamanites until they are no more called the Nephites, becoming wicked, and wild, and ferocious, yea, even becoming Lamanites (Hel 3:16). Moroni's statement that after the final great battle that the Lamanites put to death every Nephite that will not deny the Christ (Mor 1:2) implies that there were Nephites who preserved their lives by denying the Christ and joined the Lamanite society.