They be confused about what God was commanding. He was prohibiting intermarriage with unbelievers, not those of differing ethnicity.
Moses married a black women. Mirian, his sister, was struck by God with leprosy when she made a racist fuss about it.
At least two of the female ancestors of Jesus mentioned by name in the Bible were non-Jews, Ruth the Moabitess and Rahab the Canaanite. It is reasonable that others also were, especially since the Bible contains explicit laws about the procedures involved in taking women captured in battle as wives.
This is besides the fact that Israelite and Judean kings, one side of his ancestry, frequently made diplomatic marriage to foreign women, and some of them may have equally been in his line.
You obviously did not click the link to the Peterson article. I suggest you do so. You will find THIS:
The reason that one would lose his blessings by marrying a Negro is due to the restriction placed upon them. "No person having the least particle of Negro blood can hold the Priesthood" (Brigham Young). It does not matter if they are one-sixth Negro or one-hundred and sixth, the curse of no Priesthood is the same. If an individual who is entitled to the Priesthood marries a Negro, the Lord has decreed that only spirits who are not eligible for the Priesthood will come to that marriage as children. To intermarry with a Negro is to forfeit a "Nation of Priesthood holders"....
(Try this one, Dengar)