Except in all of this, the facts are true that NF heritage and ancestory is not British. So whilst we can all accept the line of questioning, even bordering on interrogation, was there, it was actually to ascertain the history.
I cant see how that is racist. Its rude yes, its crass yes. But if something is factual can it be racist to ask about it, even to push on it? SH never said to NF, "you are not British". Some of you are reading that between the lines.
To be derogatory to someone means saying things that aren't true or are an insult. I don't see any insults in the conversation, at least not overt ones by our standards. If I take something you say as an insult when its not meant to be one, that's on me.
I agree the use of the word 'really' could, if emphasised in a certain way, come across as belittling. But we still don't know whether that belittling was because SH didn't like black people or had a different view about black people. That's what racism is and we can't prove it from the conversation.