Nephi uses military terms to describe the state of his descendants in the last days. They have dwindled in unbelief, and will be smitten by the gentiles. The story of the coming forth of the words of the Nephites are presented through an interpretation and application of Isaiah 29.
The beginning of verse 15 is Nephi’s words, but when he says “yea, after the Lord God shall have camped against them,” he is restating Isaiah 29. It is not a precise copy, but one that will be adapted to the purpose of defining how the Nephite words would become a means of convincing the Jew and Gentile that Jesus is the Messiah.
After loosely quoting from Isaiah 29:3, Nephi uses Isaiah’s phrase “they shall be brought down” to indicate that his brethren would be slaughtered and therefore be buried and become as dust. That is the intent of “even that they are not.” The Nephites will perish.
In spite of having perished, however, they will have written down their words, and the prayers of the faithful will preserve that which was written.