Here the 1840 edition accidentally omitted the subject pronoun it. This reading without the it continued in the 1858 Wright edition and the first two RLDS editions, but the 1908 RLDS restored the it to the RLDS text. The omission of the it forces readers to interpret the that as the subject pronoun and to assume that there was no subordinate conjunction. Such a reading is highly unusual, if not impossible, for the Book of Mormon text (there are no examples of “that is he” or “that is him”). Surprisingly, this misreading continued through several editions in the RLDS textual tradition before being corrected.
Summary: Maintain the subject pronoun it in Alma 39:15; the 1840 omission of the it is clearly an error.