I ran the New Orleans RNR Marathon in both 2016 and 2017.
It is easy to legitimately run that marathon and miss the 5K mat. That part of the course is an out and back along historic St. Charles Avenue. The street is a 4 lane street with a very wide, grassy neutral ground in between the two sides. The neutral ground is wide enough to run TWO streetcars running in opposite directions, with plenty of space between the streetcars, and plenty more space between the streetcars and the automobiles driving on the concrete road.
Grass is easier on the knees than concrete, so alot of runners - including myself - take to running the neutral ground between the two sides of the road when running the St. Charles out and back. We are not plotting to cheat -- we just want to give our knees a break by running on grass for awhile.
Now, frankly, that makes it VERY EASY to cut the course, as there is absolutely nothing (other than our integrity) to divide those of us running the neutral ground in opposite directions. This guy could easily have crossed sides by joining the runners on the neutral ground, and then, at some point, literally just turning around and running in the opposite direction. Maybe he could have faked tying his shoe or something to help mask what he was about to do.
However, having said that, I also have to point out that it is VERY EASY to honestly and legitimately miss the 5K and 10K timing mats for the same reason. Those mats are out on the concrete streets. They don't extend into the very wide neutral ground.
I legitimately ran all 26.2 miles. Nonetheless, in 2016, I accidentally missed the 5K mat because I was running the neutral ground and didn't see it out on the road. (In 2017, I deliberately looked for it, and swung onto the road long enough to cross that mat, then went back to running the neutral ground.)
I have no trouble believing this clown cut the course, explaining the missing mat data. He just "smells bad" to me.
But at the same time, I know from my two years of running that marathon that it is easy to legitimately miss the 5K and 10K mats, if you run the neutral ground instead of the concrete street.