If it is your top choice program, I would encourage you to be very transparent about that with them. I would probably steer away from lying- if you're looking for a research-oriented career, there is a good chance that you could run into that POI at conferences or other professional settings, an you wouldn't want to develop a reputation for lying. My guess would be that they don't want to make too many offers that get turned down, so they're trying to feel out your interest in them.