I don't think that's necessarily true, especially if that applicant is within that school's average of accepted students. If the applicant has a weaker application, then I'm sure that that person is more likely to end up on the waitlist, though. But this process is a crapshoot. Never underestimate crapshoot factor in medical school admissions.
Interviews for waitlist spots do seem to happen -- but I don't think the school intentionally invites weaker candidates that they intend to at best put on the waitlist -- they just only have waitlist spots remaining toward the end of the season when the class is getting very full.