My father used to say that. Had they been left in Africa..and I am NOT saying it was good they were enslaved....but more likely than not they or their family lines would be killed off, starved to death, or worse by now in that war and disease ravaged country.
Black Americans, other than immigrants from Africa, would not exist if their ancestors had never left Africa. Every person is the result of a unique constellation of ancestors—remove any one of those and the descendants are never born. African slaves were deliberately mixed up on plantations so they would not have a common language—so slave men and slave women had children together but were from different tribes probably from widely separated places in Africa so they would never have met in Africa.