It's a tough choice for Rupp.
No any event, medal chances are proportional to event softness X flukiness.
For example, the 100 is not soft nor is it fluky. Medalling is hard unless your name is Bolt or Gatlin.
The high jump is not soft, but quite fluky. Possible medal chances for anyone in the top 8.
In Rupp's events, the 10,000 is soft as s#!t but not so fluky. Usually plays out by ability, even if relatively few great runners bother.
The marathon is not soft, but very fluky.
By focusing on the marathon, where he is likely not even top 50 globally on PRs, Rupp is showing he'd rather bet on flukes than on his ability.
Good choice for a guy who's never won a major international race on or off the track!