How to handle off-topic questions during an interview for a PhD/academic position?
Such a question at the start of an interview sounds to me like a poor "warmup" question - meant to be an easy lowball, and failing miserably. As such I think a light answer is all that would be expected - they are not asking for help, but thinking this is one you could easily answer. Your reply should demonstrate you can think on your feet, without appearing to put the interviewer down and without really engaging in an answer. For example


  I met such a variety of people there that I really can't tell you what they had in common - I guess you would have to ask the admissions office


That's probably quite truthful, while being completely evasive. And then you can move on.