If you saw a Japanese man claiming a white English man and a black Nigerian man, who had very recently moved to the country and become Japanese citizens, were in every single way as ethnically Japanese as a random Japanese man, you would rightly assume he was mad.
If he then insisted he literally couldn't even understand the counterargument, and that anyone even daring to make the counterargument - ie, that there's such a thing as Japanese ethnicity - was themselves insane and clearly dangerously racist, you would assume he was not just mad but engaged in some clearly ideologically-motivated form of intense gaslighting.
But in England...