There are two types of runners.
Those who see every race as an opportunity to run their best on that given day. Think Pre.
Those who focus on a handful of key races a year, and see every other race as preparation or gravy. If your goal is the olympics, then Oxy would be preparation. A meet after the olympics would be gravy - great to have, but not essential. Think Lasse Viren, or today Mo.
Rupp is much more of the later camp, though he does try to run fast times as part of his prep or gravy races.
Dropping out of Oxy means a couple of possible things: 1) he's focusing more on the marathon, 2) he's had a set back and is not quite ready, 3) training is not going as planned and he needs more work and less prep races.
I'm sure there are dozens of other possibilities as well. However, I doubt ducking other runners is one of them.