Otherwise are we just saying that Mexicans are lazy compared to Cubans and that is just culture? Or African Americans aren’t as hardworking as Ghanaians and that stereotype is what it is and we should just operate around it?
Yes, I think that is the author’s point. The values and habits of the Cubans who fled Cuba and Ghanaians who left Ghana are ones which lend to successful lives. These values and habits, such as placing importance on higher education and academic achievement, are partly what culture consists of.
The alternative hypothesis, if one rejects the notion of value and habit transfer, is to claim that intelligence is genetically transmitted. That opens a very ugly can of worms and to my (limited) knowledge is unsupported by scientific evidence.
In either case though, I think it is clear that the Cuban and Ghanaian example precludes the possibility of racial discrimination.
It seems to me this has deep implications about effective and ineffective policies to raise the living standards of Mexican immigrants and African Americans. If we fail to address the root cause (poor cultural practices vs racial discrimination), the problem won’t get fixed.