Well MLK was a theologian, not an economist. Many countries have capitalism and never had slavery. And while they have a mindset that isn’t “Protestant work ethic,” it’s indistinguishable in relevant part. E.g. Singapore, Japan, South Korea, Taiwan, etc. https://twitter.com/dnystedt/status/1558863974883823617
> Many countries have capitalism and never had slavery
You ignored the second part of his statement, where he claimed that since slavery ended, capitalism has been propped up by the exploitation of the poor. Those countries may not have slaves but they do have poor people.
All countries have poor people. The capitalist ones have fewest of them, because capitalism makes poor people wealthier, unlike alternatives, which make them starve and die.