Off Course Oscar was a poster who explained it a dozen times.
Bike or car. Even if he had help, there's no other way he could have done it. On a course that's easy to cut, he wouldn't need either as he still looks like a spectator walking or jogging to cheer on other runners at another point in the race by taking a short cut.
Now get rid of your extra clothes and enter on the course jogging with your number covered.
He looks back and was caught doing so in photos for what reason? To make it look like he's looking for a friend to run with, as in help out. I've done this in many marathons for several miles and then exited the course. When there's a big enough gap with a hill or sharp corner, he pulls his number up and he's now a runner in the race.
I could do the above easily and not get caught. If questioned about missing timing mats, "I don't know". Not hard people.