The Nephite nation had been destroyed. There were no more Nephites. By definition, those who remained were Lamanites, but Mormon had taken pains to note that there was no significant difference in righteousness among the peoples. For nearly two hundred years, both groups had given up their designations as opposites, and were joined in righteousness. After that time, both began to become wicked again, and Mormon once again declined to lead his people in war due to their unrighteousness.
Therefore, when Mormon spoke to future descendants who would be of the house of Israel, he would have designated them as Lamanites, but only as a category and not as enemies, nor as being unrighteous. Mormon told of different Lamanite populations who repented, and those who did became arguably more righteous than the Nephites. That is certainly the message of the Book of Mormon to modern Lamanites. They inherit the blessings, and, upon accepting Christ and living the gospel, can become “white and delightsome,” not because there is any physical change, but because they are the inheritors of the Lamanites who became white, or righteous, even more so than the Nephites.