First, in my experience with Rothbard he was a practitioner of drawing his conclusions without evidence and then cherry-picking his way to prove it after the fact. His racism long predated any of his economics. He started in 1948 as a backer of the racist States’ Rights Party of Strom Thurmond.

In my experience in the U.S. and living under the last days of apartheid in South Africa I often ran into racists—rarely were they libertarians—who argued blacks were less competitent. But that always raised the question: If they were incompetent for a job then why did you need legislation reserving the job for whites? In South Africa the Job Reservations Act was in place to stop blacks from taking the jobs the racists argued they were unable to do. If they are unable then you wouldn’t need laws in the way. If anything, you have to conclude that the people who were being protected from competition—white unionists—were the ones who were incompetent as they were the ones needing protection.

As for meritocracy, there are many ways ot looking at merit and intelligence is put one of them. In fact, most studies I’ve read on the cultural values in a successful economy rarely discuss intelligence at all. A hard worker with an average IQ, regardless of race, is likely to do better than an intelligent individual who thinks the world should be at his feet because he is intelligent.

In my experience there are serious errors that intelligent people make all the time about productivity and effort. I would argue that the values of a productive economy include hard work, honesty, trust and a myriad of social values which are not directly linked to intelligence at all.

The role of cultural values—over intelligence—is discussed here:

Values Matter: Liberty and Culture

https://medium.com/the-radical-center/values-matter-liberty-and-culture-761c2ad235ca