If it says the DNA match is Nigerian, what that means is that there is a DNA match to one of the 250+ different ethnicities of Nigeria. (How do I know this? I’m Nigerian).
Now even though Nigeria’s population is so diverse, only a few of our ethnicities are nomadic. (Fulani primarily).
Controlling for location, an African American whose DNA shows a match located in Nigeria is almost 90% more likely to have a match in Southern Nigeria as opposed to Northern Nigeria (this is because the slave trade / most of the slaves that were captured and transported to the United States and other places came from Southern Nigeria ).
In other words, the chances of an African American having a DNA match to Hausa ethnicity (which is the largest ethnic group in Northern Nigeria) is less than 10%.
Conversely, the chances of an African American having a DNA match to Igbo, Yoruba, Bini, Efik, Igala, Ibibio, Ijaw or Itsekiri ethnicity (assuming they have already been DNA matched to Nigeria in general) is almost 90%.
When you then factor in the fact that Igbo and Yoruba account for the vast majority of all the people in Southern Nigeria, an African American who has been DNA matched to Nigeria in general has an extremely high chance of being either Igbo or Yoruba and a moderately high chance of being from one of the other southern ethnic groups.
So there you go.