Verse 20 makes it clear that the story told in verses 17 and 18 was on the Nephite copy of the brass plates. It is not in our Old Testament. The story begins with a people who “had waxed strong in iniquity and abomination; they had all gone astray.” This is the parallel to the people of Ammonihah.
Through Melchizedek’s great faith, and his understanding of the coming redemptive act, he preached repentance to his people, and “they did repent.” Alma, presumably through that same ordination and priesthood and with that same understanding of the coming redemption, is preaching to the people of Ammonihah. Clearly, the message is that they, too, should repent.