I had always understood it was something close to that, but not exactly. What expedia really has purchased is an option on rooms. It pays a certain amount -- say $50 per room -- for the right to buy the room, and to use the $50 toward the purchase. If the options expire unused, expedia is out the option price, and disney can rebook the room.
Expedia's cancelation policy is pretty liberal. You can cancel up to 5 days prior to the visit (subject to a 25 dollar penalty); you also only get hit with a one night payment for later cancelations.
I imagine that expedia doesn't do so well on getting out new reservations for rooms that get canceled 5 days before the trip. Expedia would be taking a pretty big gamble, which is why the option system sounds right to me.