Mormon uses these two verses to create a contrast. The Nephite/Lamanite contrast has always been part of his text, but now Mormon uses that conceptual tension to turn the associated principle of righteousness around. The Nephites and Lamanites are still separate, and still opposed, but now the roles have shifted with respect to the gospel. As the righteousness of the Nephites fades, the righteousness of the Lamanites increases.
This literary device allows Mormon to show the continuation and overarching value of the gospel. Even when the Nephites abandon the gospel, it is still true, and its value may be accepted by the Lamanites. The next use to which he will put this shift in righteousness is to have a reason for the continuation of the Lamanites when the Nephite polity is destroyed. The Lamanites will achieve some protection under Lehi’s promise because they have now become righteous.