There is no question Tesla, like everyone else, uses an array of methods to encourage sales. It is a business.
The pre-registration of cars is, in my opinion, unusual for Tesla but does happen. They have no independent dealer trying to artificially boost sales figures and I'm pretty certain they don't pay sales commissions. Having bought two only, but both times the advisor I was with in the showrooms told me that placing my orders online was the best way and they encouraged me to do it that way when I got home. Seems odd to push off a potential commission both times?
Pre-registration doesn't achieve much for them. They are required to account for sales using US GAAP. A vehicle pre-registration does not get booked as a car sold. Perhaps sometimes a buyer backs out before collecting a car or financing falls through at the last minute? There are many possible explanations. Cooking sales figures seems an unlikely one to me.