Rhetorical: Jacob's art in speaking comes into play here. He creates a set of symbolic reversals on his themes. He has spoken to the pure in heart, and now addresses those who are not pure in heart. This direct opposition is followed by other oppositional sets.
He first contrasts the cleanliness of the pure in heart to the filthiness of the "not pure in heart." This contrast is implicit rather than explicit. His next contrast is between Lamanite and Nephite. Jacob clearly understands the categorical use of the term Lamanite as indicative of the opposite of Nephite, where the standard social determination is that Nephite = good and Lamanite = bad. Jacob turns this cultural definition upside down, and declares (that in this one thing) that the Lamanites are superior to the Nephites. He gives a reversal of expected usage, now applying filthiness to the Nephites, an thereby implying a cleanliness for the Lamanites, a complete reversal of standard expectations.
His final literary parallel is to place the future "destruction" of the Nephites as a mirror of the "destruction" of the pure in heart. Just as these pure women will be destroyed by an outside force, so will these Nephite men be destroyed by the outside Lamanites (unless they repent).