I'm afraid your chronology is off. There were both European colonies in the Americas and African slaves in those colonies in 1623. Spanish and Portuguese settlements dated to the late 15th and 16th centuries - indeed, Spain had built a substantial empire encompassing much of the Caribbean and Mexico by 1519 - and both soon began employing African slaves to supplement indigenous labor. Jamestown, the first permanent English settlement in the Americas, was established in 1607 in modern Virginia.

I suspect it is to this colony that King was referring. Jamestown was founded for the purpose of mineral extraction, the idea being that since Spain was digging huge amounts of silver out of Mexico and Peru, so could England. Virginia turned out to be notably poor in precious metals, and the focus of the colony switched to the production of tobacco, the plant having been brought over from the Spanish colonies. The English were largely reliant on the labor of indentured servants for their first few decades in Virginia, but they began supplementing their labor with African slaves beginning in 1619. In that year, an English privateer arrived at Jamestown bearing "20 and odd" Africans taken off of a Portuguese slave ship. Their status remained murky for some decades - at least some were treated as indentured servants for several decades - but laws passed in the 1660s established racial slavery within Virginia.