Limhi sees the arrival of Ammon as a means of entering into the covenant that Alma had made with God. Indeed, as indicated in verse 32, they had already made the covenant in their hearts, but they were awaiting the final act of that covenant, which was baptism. Having seen Alma and his church, they wanted that same benefit for themselves, and presumed that Ammon would be able to provide them the benefits of Alma's baptism. Ammon declines. The official reason is that Ammon considers himself "an unworthy servant."
The Limhites have already delayed baptism because "there was none in the land that had authority from God." They presumed that Ammon would have it. It might be that Ammon did not feel that he did. Of course we do not know the intricacies of priesthood in Zarahemla, so we do not know whether or not Ammon might otherwise have performed a baptism. However, there is another possibility that becomes clearer in the next verse.