Rhetorical: These words must have bitten like a venomous snake into the hearts of the Nephites. Centuries of supposed superiority over the Lamanites are now turned, and Samuel declares that the Lamanites will be more favored of God than the Nephites. This is the ultimate reversal of all that the Nephites would ever have believed. Even if they had rejected their belief in the Atoning Messiah, they certainly maintained their air or superiority over the Lamanites. This must have been the ultimate insult.
Of course it was insulting only because the Nephites were not able to be humble, recognize the truth of it, and do as Samuel entreats: repent.