I don't see any proof of it being an addon car. And I know from the author itself it is not. That said, there was a tricky way to "add" cars to the game. The car mesh is an entirely different one for backward tracks and/or mirrored ones, I can't remember clearly. So theoretically you could double the car amount at least if you take the job of modding every car in the came.
For a "proper" car addon, you can't in single player. The menu and its resources are somehow limiting to do it. But there is an exception where you don't need the car select menu at all: Multiplayer! Yeah, there is a simplified one, but with the console and the /request *car folder* command you can play with an addon car, as long as you edit cars.ini to be recongizable by the game.