Except that White Supremacy pre dates the chattel slavery of the Atlantic Slave trade. The whole concept of "Race" and "Whiteness" was created to justify the dehumanization of African people to take them into bondage. White Supremacy allowed the enslavement of African people.

Slavery existed before the concept of race, but it was typically associated with warfare between neighboring groups. There was supremacy too, but it was "Greek Supremacy" or Spartan supremacy, or Athenian, Roman, Mongol, Chinese, etc... supremacy. There was not a concept of racial superiority until "White people" wanted to take Africans out of Africa to make them slaves on the sugar and cotton plantations of the new world. The Portuguese were the first to push the concept and the Dutch were the first to make the trade lucrative, bringing the first slaves from Africa to the Americas. Columbus took "savage" Taino people from the new world to Europe, but the trade never really got going in that direction.

White Supremacy is the underlying evil from which slavery was justified. Native Americans were considered "Almost White" by none other than Thomas Jefferson. He thought that Natives just needed some culture to become fully White. That is, until the colonists/Americans wanted the land which the natives inhabited, then they became Savage Red Men, and were justified for extermination.

If you look back at historical cartoons, you will see Natives, Mexicans, Filipinos, and others drawn in a Black African or Native caricature, It was the not to subtle way of communicating the racial inferiority of those people which justified taking of land or subjugation. It is easier to enslave and subjugate people, or steal their land if they are inferior and sub human.