Let us look at the end of Helaman 8. After the corrupt judges tried to get Nephi arrested, they pushed a little more. One of the criticisms that these people had raised against Nephi was that he was only one person witnessing against the people. Nephi took the occasion to speak about prophets and prophecy. Starting in Helaman 8:16, he began invoking many prophets who had spoken the truth, and had spoken as single voices. Nephi offered the examples of Moses, Abraham, Zenock, Zenos, Esaias, Isaiah, and Jeremiah—who stood as witnesses of God, even when they were alone.
Just as Jeremiah had prophesied the destruction of Jerusalem, Nephi was now prophesying of the Nephites’ destruction. We know that Jeremiah was a true prophet because of Lehi’s words. After Lehi departed into the wilderness, he received a vision validating the destruction of Jerusalem. In addition to Lehi’s dream, the Mulekites also attested to Jerusalem’s destruction. The founding story of Zarahemla is the story of Mulek coming from the destruction of Jerusalem. Here, Nephi uses that as evidence for the truthfulness of prophecy.
Book of Mormon Central, “Why Should Readers Pay Close Attention to the Mulekites? (Omni 1:19),” KnoWhy 434 (May 17, 2018).
Book of Mormon Central, “Why Did Nephi Rely on Earlier Testimonies of Christ? (Helaman 8:22),” KnoWhy 179 (September 2, 2016).