Morianton becomes an exception to the model of righteousness leading to prosperity and unrighteousness leading to poverty. This tells us that although Moroni is molding history, he is not forcing history into the mold. When there is an unrighteous king whose reign is prosperous, we are given that information. Of course the accompanying moral implications are missing, but the historical point is retained.
Chronology: The average-reign chronology would place the reign of Morianton from approximately 770-740 B.C.