It's been some time, but as far as I recall, TS was addressing the claim that inner city black ghetto culture (gang culture, really) is the authentic black culture which came over with slaves from Africa. Often with a follow up claim that therefore it needs to be accepted or else its racist. It's usually not expressed quite the same way today but you still see the general idea in claims that, for instance, black people shouldn't be expected to be on time, or black kids in school shouldn't be expected to learn "white" math.
Sowell's claim is that is not the case that this is authentic black culture that came from Africa. His argument is that the slaves largely lost most of their African culture within a generation or two, and often what replaced it was the culture of the poor whites around them - what is sometimes referred to "Cracker" culture in the US. These people were largely descended from Scots-Irish pioneers, many of whom had been part of the underclass back in the old country. They were rural, and were not what you would consider diligent working farmers, but the kind of people who drank and sang a lot, were emotional and thought a lot of their honour, fought a lot, and only did the bare minimum of work to get by. At least that was the stereotype of them and there are some sociological studies that suggest there was some validity to that stereotype.
According to Sowell both these poor blacks and whites ended up adopting much the same religion, social values, way of life, and even language elements. Many moved North eventually to cities in mass migrations where they were often not well recieved by Northerners whatever race they happened to be.
Sowell's claim is that the problematic elements of ghetto culture are what remains of these people.