Former head of product there here: no, we didn't do this kind of identity verification. That would have prohibitively damaged people's willingness to actually do the interview. We did use various means to try to identify people signing up multiple times, and we caught plenty of people trying to duplicate themselves that way, but you didn't have to physically go to a center to do our interview.

I've considered something like that for my current company, which is doing basically the same thing, but:

(a) this has not, in practice, been a problem for us in identifying good candidates

and, far less importantly:

(b) you need very high scale to have interviewers everywhere that candidates are OR you're paying extra for a third-party controlled environment

(c) scheduling and cancellations become more difficult and costly respectively