What Jimmy said.
To be pedantic, they are state machines but they sure aren't simple. If you want to merge a state machine perspective with the pyprocgame mode philosophy, it probably breaks down that each mode is a state and the transitions between the states are probably composite actions (not just individual switch hits). Each mode itself may itself be a separate state machine depending on what the mode does, and here the transitions are more simple, like switch events and timers.
To drive home the point about the mode perspective, there's lots of docs on the SkeletonGame website, including this.
http://skeletongame.com/mode-basics/
It's not clear that it's beneficial to get too locked into the state machine concept for SkeletonGame, but what is clear is that I've gone way too deep into technical details on a club thread...