In his editing for the 1837 edition, marked in the printer’s manuscript, Joseph Smith changed the original conjunctive phrase than as though to than if. Ultimately, the 1837 edition and all subsequent editions read than though—that is, the as has been deleted.
It is quite possible that the use of than though in the current LDS and RLDS texts is simply an error that originated in the 1837 edition. Elsewhere in the text there are examples of as though without any preceding than (17 times). In addition, there are 51 occurrences of as if, but none of these have a preceding than. There are no occurrences of than if (Joseph Smith’s manuscript emendation in Alma 24:30), although there is an occurrence of this usage in a revelation that Joseph received near the end of the printing of the first edition of the Book of Mormon (in March 1830):
Finally, we should note that there is nothing really wrong with the original than as though here in Alma 24:30, the extant reading in 𝓞 and copied as such into 𝓟 and into the 1830 edition. According to Literature Online , there is evidence for this usage in the 1800s (at least):
Summary: Restore in Alma 24:30 than as though, the reading of the earliest textual sources (the two manuscripts and the 1830 edition); examples of this complex conjunctive phrase can be found in the English of the 1800s (at least).