Although Nephi is not quoting Isaiah, he is alluding to Isaiah. Verse 18 alludes to Isaiah 29:5, which says: “Moreover the multitude of thy strangers shall be like small dust, and the multitude of the terrible ones shall be as chaff that passeth away: yea, it shall be at an instant suddenly.” This is not a complete quotation, but a modified quotation. The essence and much of the wording is preserved, but it is recontextualized into Nephi’s vision of the future, rather than Isaiah’s original context.
The image of speedy destruction will be applied to the future Nephites who will “dwindle in unbelief and be smitten by the gentiles.”