Aren't waitlisted people usually notified the same time as admits, though? I feel like that happens a lot of times on the boards, in which case if you're not on that waitlist, then you're relying on the original acceptances to bail, then the waitlisted people to fall through, then you might get offered admission. Probably unlikely right, if you see both admits and waitlisted people notified at the same time?
I mean, there is some logic to being a graduate department and "holding on" to some people without dealing with the hassle of telling them they are on a waitlist, because meanwhile those candidates won't take offers from other schools or give up unless they've heard. But I don't know if that's me being just optimistic (I'm waiting for two schools still, but admits went out two weeks ago)....