> I just mean that if you put 2 experienced people talking about a topic they both know, it should be pretty easy for both (or at least the interviewer) to get a rough understanding of the level of the interviewee.

Well, in my experience it's not, at least not in a "hostile" context. Most technical people are used to assuming good faith in technical conversations, and there are some very smooth bullshitters around; it's easy to construct a verbal facade that only falls apart once you ask someone to actually code.