Listener ,
I can see your point. The only parallel I can draw is that the History candidates have to do a similar thing with a piece of text. This is , admittedly, I think designed to be a text no-one has studied but the principle is sort of the same, i.e. from something you don't know a lot about you have to question the source for reliability etc. . So if you don't know about UK radio stations , then the I am guessing a valid line to take would be
DD - Classic FM = fairly reliable
Interviewer - but who listens to Classic FM?
DD - I do , but I don't know the target audience for Classic FM as don't live in UK, so it is difficult for me to say .
At which point the conversation may have gone down the track of - well from what you have read , heard on the radio , what do you think the target audience is etc. ? Which would have been interesting.
Listener , I am speculating wildly here, as what I know about music could be written on the point of a baton , but just trying to think along the lines of your question. I would doubt that an interviewer would be being derisory , nor that he or she would think listening to Classic FM is a Bad Thing per se . If there was any mistake , my thought would be it was Dd saying the source was reliable without any backup other than her own experience (1 person).
I think I am going down the lines that Tantalising has so much more succinctly done.
Your post though resonates with me. It takes a deal of confidence in an interview like that to admit you don't know something , or don't really understand the question. That is the one piece of advice I gave DS. If you don't understand what they are asking you - just say so . Luckily he only had to employ it once 
Anyhow , not sure if that helped but I do wish your Dd well and hope she is not too deflated by it.
Hocus.