I looked at the Pre-Order agreement, it is only about 2 pages long. I think the correct sentence about applying the amount to the purchase prices is this:
"When you take delivery of the Vehicle, we will provide a credit to the final pre-order price of your Vehicle equivalent to the amount of the Pre-Order Payment you paid."
So it appears that the price is locked in when you pre-order the car, but can change if you modify your order. And the pre-order payment (not a deposit) applies to the price of the vehicle. While the "locked in" part is my verbiage, I infer it from this on the Reservation Page:
"Please note that changes to your order could impact delivery timing, insurance and price."