I don't know. I think Duke, Vandy, Northwestern, and Stanford (all private institutions with a strongly academic focus, but residing in a Power 5 conference) are every bit as much a stepping stone to "real Power 5 jobs" as any Group of 5 school.
Whether a guy is interviewing from Duke or from UCF, I don't think anyone much cares. They just look at what the coach was able to accomplish, how well he performed on all the different metrics required of a head coach. And if he did well, then he has the chance to interview for the really top jobs.
Steve Spurrier, Jim Harbaugh, and James Franklin have all used those schools as springboards to bigger head coaching jobs. Spurrier in particular did it from Duke.
So there's no backwater curse. Duke and the other schools like it don't often get a great young coach...but when they occasionally do, there's no curse to starting there. Proven.