This is a trivially easy question to answer and it shocks me why anyone is taken by the comparisons between immigrants and native-born Black Americans. Immigrants, especially immigrants from countries that don't share a land border, are almost always systematically economically different from both the average person in the society they're emigrating from and from people who happen to share the same racial group in the society they're immigrating to. American-born black people descended from the enslaved are not the same group of people as recent Nigerian immigrants. For the former group, the poverty, employment, wealth, life expectancy, and a thousand other gaps relative to white people have been extremely consistent throghout American history. You can go back as far as you want. I think what needs to be better justified is that the persistence of that gap very suddenly changed causation to culture and welfare and wasn't because of the long history of actively destructive economic oppression and exploitation followed by a complete lack of redress. Even setting race to the side, economic position is very intergenerationally sticky in the US—children tend to end up in a very similar economic position to their parents.