In the case of immigrants, it's more of a "truism" that Africans work harder than American blacks. Maybe it's true, I don't know. But usually the first wave of immigrants from anywhere are the smarter ones who had the initiative to get out of the bad conditions back in their homeland. However, that changes as time goes on and an immigrant community forms. Once that happens, more average and typical people from back home start showing up and things balance out.
BTW, the Somali Bantu certainly aren't outperforming American blacks. They've practically bankrupted some small towns such as Lewiston, ME with their welfare demands. But, you see, these aren't people from the upper tier of their nation who took the initiative to come here and escape poverty. They're refugees from war, meaning they're average folks from their homeland.
And, of course, all this begs the question of why Africa isn't performing well, or at least performing at a better level than American blacks. As poor as Detroit may be, it isn't as poor as Cameroon or Zaire. If African students and immigrants who come here and outperform American blacks are typical representatives of the African population, then why is Africa mired in poverty? And imagine where Africa would be today if the Colonial powers had just decided to sail on by, and had ignored the continent these past few centuries.
The discrepancy we see in IQ scores between the average African American and the average European American is mostly due to culture. If it was genetic one would expect that the American blacks who are of mixed African and European descent would do better than those straight out of Africa blacks, but they do not.