First of all not all African American have double digit Nigerian test scores. Some have double digit Cameroon/Congo test scores, but I have almost never seen an African American that did not have a double digit Ivory Coast/Ghana score. The reason they come back double digit for Ivory Coast/Ghana is because Ancestry doesn't test for Sierra Leone, Guinea-Conakry/Bissau or Liberia. It is not even in dispute that most African Americans trace back to those areas, because they brought vestiges of stuff: like Blues music which is from the Sahel; Islam which still exist in AA churches right now, except nobody ever notices it; and customs like "playing the dozens" or "signifying" or "singing on each other" which is called "senankuya" among the Mandingos, which is where African Americans got that tradition from.
Another thing; leaving from the Bight of Biafara does not mean that the slaves were Igbo or even from Nigeria. The Bamoun and Bamileke people of Cameroon were also taken through the Bight of Biafara. The Sara people from Chad were also taken through that area. DNA testing is showing that Cameroon was grossly underreported as a the place of slave captures. Over half of African American men have Bantu genetic markers, which also indicates that they are from Cameroon and to the areas South and East of Cameroon.
African American Roots: What Genetics Can Reveal - 23andMe Blog
Cameroonian Americans - Wikipedia
Finally, only a very small percentage (something like 5%) of slaves were even brought to the USA. The vast majority of the slaves went to the Caribbean and South America. When the USA ended the importation of slaves directly from Africa into the USA in 1807; all new slaves into the USA were brought in from the Caribbean. That is likely how most Nigerian DNA in slaves got into the USA, because those slaves from the Caribbean were being bought into the USA.
Most of the 23andme records I've seen always show Nigeria and most times it's higher than 10%.
However, from the records I've seen - it's different (I'll find the slave records for the US and post it later). And most of the research papers I've read from African American academics/historians, including Henry Gates, also support this claim. The claim you're making for Ivory Coast/Ghana can be made for Benin/Togo since there are indigenous Yoruba populations in those countries that are never accounted for, hence the DNA records of a lot of Nigerians on 23andme always have the Benin/Togo markers.
Blues and all the other Sahel culture you're alluding to came from Louisiana, which wasn't part of the US, until the 19th century, when Napoleon sold it to the US (you can't deny the fact that most of it came from Kongo Square in New Orleans). And slaves taken to French colonies were allowed to keep their culture, unlike the US, where slaves were stripped of their native cultures. How about cite cultures that came out of a place like Virginia, since Jamestown was the first place slaves were taken to in the US, and a large bulk of the slaves were also taken there? I'm sure you won't be able to find any culture to cite, since they were all stripped of their native cultures and it's always easier to cite the cultures that originated from Louisiana were slaves were allowed to keep their culture.
Bantu genetic markers doesn't necessarily mean Cameroon since a lot of Nigerian groups in Eastern Nigeria, close to Cameroon, also have Bantu markers due to the fact that Bantus expanded eastwards/southwards in Africa, from there. And the slaves taken from Kongo Kingdom were Bantus since the slaves from there also form a large bulk of the slaves taken to the US.
Yes, I already alluded to the slaves that were smuggled into the US from the Caribbean and Brazil (I believe Brazil was still actively involved in the slave trade till 1907). But a lot of slaves from Nigeria have always formed the bulk of slaves taken to the US.
Anyway, this is beside the point. A lot of Aframs, especially the ones in the South where most Aframs live can fit anywhere in West/Central Africa and you won't be able to differentiate the people. And some even look more 'African' than the average West African kid born in Europe. So, the argument that all Africans have a certain look is redundant, if you ask me.