Since this was before (or during) the invention of white people, I'd say it's just how exploitation works. Anthony Johnson wasn't trying to be white because the concept didn't exist yet, and for the same reason, he didn't care that John Casor would be called a fellow black person today. Johnson just wanted to make more money from the man he had bought, so he did not want to treat Casor like an indentured servant.

But I can agree it's how the roots of white supremacy were laid.

if you want to read more about it: https://www.smithsonianmag.com/smart-news/horrible-fate-john-casor-180962352/