In fairness she hasn't said she doesn't have Nigerian heritage? You can Be British, have British nationality and consider yourself English but also know you were born in America or Nigeria or that your parents came from there. Boris Johnson was born in America but gave up his nationality there (in his case because of tax). He also has a Turkish grandparent but while I think it would be reasonable to point out he has Turkish heritage, doing it as a gotcha in response to him saying he doesn't consider himself Turkish/American would be unreasonable. I assume he doesn't "identify" as American. It's up to him
I am partially Irish but wouldn't identify as Irish because to me that would be weird and cringe (even though Irish people are great or that other people cant.) if someone wanted to say "Persephone has Irish heritage" then that's technically true.
I dunno, just feel a bit bad for her being nitpicked on this.