That makes sense, but I'm curious if there are any other airports that are coterminals with two airports that aren't also coterminals with each other?

UA used to (and probably still does) treat SAN as coterm with LAX. I used to do LAX-DTW-SAN annually on awards that weren't supposed to allow open-jaw. At least once I had to talk to a phone agent who had to check with a supervisor who then confirmed it. Unless traffic is bad, SAN is well within the driving range (time-wise) of people who primarily fly out of LAX. So it wouldn't be irrational for AS to treat SAN as part of the LAX coterminal family - I don't think they offer any flights between any of the pairs.