That's one interpretation. Another (mine) is that with the X introduction imminent, there is a very real possibility that 2015 Q4 Model S sales could be cannibalized by people seeing the X and deciding to get in line for a 2016 delivery. That would make this a very positive proactive move to manage the business, before there is a problem.
Note that this incentive doesn't apply if you sell a Model X, only a Model S. It also ends Oct 31st, meaning placed orders that can be delivered before EOY. This also works in the states where Tesla cannot legally conduct sales activities.
Brilliant, imo.