The strongest theory would have to be checkpoint-to-checkpoint by bicycle.
One could leave a windbreaker and sunglasses with the bicycle, and along with a helmet, it would provide more than enough of a disguise to get from point-to-point without incident.
There are plenty of riders around a marathon course cheering for runners. A cheater would be indistinguishable in the crowd. Heck, one could probably ride along the sidewalk right next to the course, or even on the course. As long a the cheater dismounts far enough away from their entry point, pretends to be part of the crowd, and doesn't draw attention, absolutely no one would take notice.
Anybody see any bicycles around the course in the race photos?
I was initially thinking this theory would still amount to a considerable amount of physical exertion, but bicycles are equipped with motors, so it wouldn't take much effort at all...
This particular cheater has "entered" a number of triathlons, so one would assume he should be fairly comfortable on a bicycle.