He is also a founder at Tesla, as when he entered the round A financing Tesla was just three guys, some networking, and nothing more. Even the "Tesla" trademark and logo registration was made by SpaceX people. He didn't found his own car company with just J. B. Straubel (another Tesla founder who was being paid by him to develop electric car batteries at the time) because he thought it would be better to cooperate with that other group that was was inspired by the same idea and he though he would be able to concentrate more on SpaceX that way. He was wrong and had to take the CEO position on Tesla later to avoid bankrupcy, move the Model S design headquarters to SpaceX, etc. And it's now nothing like what it was when he first put money on it.