The difference between the Lamanites and the people of Ammonihah lay in what they had been taught. In verse 17, Alma has indicated that the Lamanites had believed in false traditions of their fathers, and that explained their continued resistance to the gospel. However, the people of Ammonihah had been taught correctly, at least at one time. Therefore, they are now in rebellion against truth, and that invokes the negative aspect of the promise of the land.
The people had said that they didn’t believe that their city could be destroyed in a day. Alma lays out their sins which would justify the invocation of the negative aspect of the land’s promise. He had begun his discourse by speaking of Jehovah saving his people. Alma is indicating that Jehovah will not protect them. Although Alma hadn’t previously declared how the city would be destroyed, in verse 19, he appears to promise that it will be at the hands of the Lamanites. Indeed, that is exactly what will happen.