I like radical (in the radux sense) feminism's philosophy of the identity here. You do not get to truly define a group if you are not part of it. I think it would have been way out of line for Nyad to tell Oprah that she wasnt a black woman as she thinks of it, rather her own definition of a black woman.
Nyad will tell you what she is. You listen. The end.
There's no "well you're not what I would call...."