So the leading theories are:
1. He started at the back with the walkers in order to stay under the radar and not seem suspicious. He took a photo of the crowd, a red herring, and does not take a selfie this time so no one could identify him should incriminating photos surface. The gun goes off and he triggers the start mat an eternity later. He then walks over to the finish area and waits for the right time to trigger the finish line mat. In Rossi fashion, he screws up the math and triggers the mat too late for a sub 20 result.
Potential flaw here is that we haven't seen any pictures of him finishing.
2. Start goes same way as above, except he hands his bib to another runner after the start to carry it to the finish, thus they both post different chip times. This would be corroborated if we review sequential finish line photos and he does not appear.
3. He start all he way in the back and actually runs through the start and part of the course, cutting a majority of it then resurfacing near the finish to run through the finish line. Screws up the math.
Right now I'm at 70% for 1, 20% for 2, 10% for 3, and 0% for him actually running the whole thing.