When one cannot imagine how someone could think x or y, one doesn't understand their viewpoint yet. I think it was someone like John Stuart Mill who said that, though more lucidly.
If media had not told us at the time of her and Harry's engagement I would not have known MM was mixed race until I saw pictures of her parents. So I can imagine and understand someone not knowing it.
And when I did realise I thought "So what?" Having read this thread I now realise that the what of the matter is that there exist racists who will use any chance to hurl abuse at other people (I think DB was not doing that). I can't help thinking that perhaps we shouldn't focus on a person's race. Instead of ramping up the intersectionality, why not ramp it down? That's what Martin Luther King and others like him wanted. We seem to have gone the opposite way.