The Nephites certainly know of the conversions of the Lamanites to the Nephite gospel, and it is that knowledge that Nephi uses here. The Lamanites have received the gospel, and are keeping it. The Nephites had received the gospel, but have rejected it. This places the Lamanites in the more righteous position. Nevertheless, Nephi continues to hold out hope against the dire prophecies by noting that they may still repent and avoid the consequences.