The printer’s manuscript had the negative not in the that-clause following the verb dispute. The 1830 typesetter accidentally, it would appear, dropped the not. The point of this passage is that Nephi’s audience knew that Jerusalem had been destroyed and that all the sons of Zedekiah had been slain except for Muloch. Until the Nephites discovered the people of Zarahemla, they had no confirmation of Lehi’s original prophecy hundreds of years earlier that Jerusalem would be destroyed (see 1 Nephi 1:13). Here in Helaman 8, Nephi is giving his listeners an example of a ful- filled prophecy and rhetorically asking if they really believe it was not fulfilled. Note especially how the parallel yes-no question that follows also uses the negative not within its that-clause: “will ye say that the sons of Zedekiah were not slain / all except it were Muloch”. (For discussion of the name Muloch rather than Mulek, see under Mosiah 25:2.)
Part of the difficulty with this passage may have resulted from the unique way in which the verb dispute is used here in Helaman 8:21: there are no other examples in the Book of Mormon text where dispute is followed by a that-clause. Yet there is nothing wrong with the original text (“will ye dispute that Jerusalem was not destroyed”), the reading of the earliest extant text (the printer’s manuscript). The critical text will restore the not here.
Summary: Restore the original not in the that-clause following dispute, thus making the larger passage consistent in its use of not.