Narrative: Nephi locates this apostate world in the last days by associating it with Christ’s second coming. Of course, the Book of Mormon came forth before the still-awaited second coming, but the overall context is events pertaining to the last days, particularly the Messiah’s triumphal entry as predicted by Isaiah.
Nephi is not outlining an accurate historical chronology but a condensed prophetic future. Thus, it is true that the apostasy of the world precedes the final coming of the Messiah and that the Book of Mormon’s emergence follows the apostasy. That much correlation is sufficient for Nephi.