My cousin, a committed ultracyclist (5-figure annual mileages, PBP, the whole bit) rode a century in exactly 5 hours this summer. Clock time and rolling time, since he didn't get off the bike. So yeah, it can be done. Not sure about the mere mortal part.
As far as your variables, obviously he's got the training, and I know he's got aero bars. He was riding solo, but I think he found groups to draft with much of the time as well.