The "brainteaser" question type in the survey is just another type of question. If you've seen/practiced Fermi estimates, it's pretty rote. If you've never seen/practiced Fermi estimates, it will sound like a horrible trick question. I wonder how well it would work to begin with "are you comfortable with Fermi estimates?". If so, go ahead and ask. If not, _teach_, do a couple together, then have the candidate do one on their own. The technique isn't hard, it's just surprising that it can work and work pretty well if you haven't already been exposed.