Yep it was a sexist world back then, but Obozo’s daddy was a british subject and that makes him a british subject. You have to make a positive statement to reject British citizenship.
A country that won’t allow dual citizenship has no affect on British status if it grants its own citizenship unless that British citizenship is overtly denied in a manner that Britain recognizes.
A child born abroad with potential dual citizenship does not need to make a formal election of US citizenship at 18.