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.