While in general, there are sometimes people who feel put upon and don't feel like answering questions and/or having that difficult discussion of race, a job interview isn't the place to have it.

The power dynamic is too skewed for you get anything like a real answer to your question. If the question was designed to see how the interviewee handles pressure than that's a shit move to do.

Either way it's a terrible question. Also, it's an unfair one because other white applicants aren't getting this question. By definition, there really isn't a way for a white interviewer to be able to judge how well the interviewee responded to "Why isn't it OK to say nr". It can easily end up feeling like a personal judgement or worse a judgement on a person's entire race of which there is no way they could represent every perspective.

The fact that it was in a job interview just shows the that interviewee either didn't understand the context of the question which is strong evidence that they are probably a bad manager or they didn't care, which is even stronger evidence of a bad manager.