Here is how i think they will defend it. Foles gets his key pretty much entirely off how the LBs react. There is no time in this play to read a number of keys. Unless there is something on the pre-snap that makes him want to pass before even seeing what the LBs do that is how he determines it pretty much every time. Maybe he will instead of the LBs focus on the SS in some plays depending on what the particular run option is. In either case he only has time to read 1 key per play after the ball is snapped.