I believe a lot of what keeps black people down can be attributed to culture. I don't think he's wrong about that. I think he just ignores the factors that led to the formation of that culture to begin with.
And data is great, but it can be used to misrepresent without truth. Again, you can probably show that certain problems arose when certain policies were passed, but correlation isn't causation. The impact of racism is kinda hard to show through data sometimes too because "racism" often isn't captured in the records.
For example, think about the creation of this country's interstate/highway system.
America's interstate highway system cut through the heart of dozens of urban neighborhoods.
www.history.com
Several prosperous black or majority black neighborhoods were destroyed because of that project, and highways in some cases were designed specifically to go through black neighborhoods because planners wanted to get rid of those neighborhoods anyway. When you look at some of those areas, the black people were basically herded into areas that are now looked at as hoods or ghettos. I bet you Sowell's stats don't account for things like that and Sowell never brings those things up or discusses their impact. He just acts like our problems are either our fault or the fault of liberals.