If that's the filter they want to use -- the time to apply it is before inviting the candidate to take 3 days out of his/her life to fly across the country for an interview. If it's such a huge "ding" for them that a candidate doesn't come from a certain set of preferred schools -- fine, don't invite them for an interview. It's really quite simple.

Of it's a "ding", but they want to give the candidate a "shot" -- that's fine too, but there's no need to blatantly neg the candidate right to their face. It serves no purpose; it's just uncivil and unprofessional. (No purpose, that is, other than to give the interviewer an ego rush, and thereby provide a temporary defense of sorts against their own very deeply rooted insecurities. That, and to basically deep-six the candidate's performance, and nullify whatever enthusiasm they might have had for Airbnb during the rest of the, by that point, manifestly pointless "interview").

And the fact that such allegedly highly educated people would so quickly resort to numbskull behavior like this suggests that maybe's it's not such a good filter, after all.