People seem to love this B.S. don't they? If it's just country of birth, Dillian Whyte's Jamaican so. Could have sworn he was British born and bred listening to Sky Sports and the like. He's probably had a good few full English breakfasts now, so he might even consider himself a fully-fledged Englishman! I mean things have changed now, dudes, nationalities are more fluid now.
So it is an abstract concept, and up to the individual. Makes sense to me. The other alternative is to say we are all Ethiopians. Either/or I am not bothered.
To me, they can be several nationalities, I would consider Joshua British/Nigerian I would consider Lewis British/Canadian/Jamaican, Tszyu Australian/Russian/South Korean