For that matter, he's also not a founder of SpaceX, and even calling him co-founder of PayPal has a lot of clarifications around it. If PayPal had gone ahead the way Musk had wanted, it could easily have sunk the company into another joke of the excess of the dot-com boom of the 90s.
As much as I am loath to say anything about Musk, but he did actually found SpaceX - although that's probably the only company he founded (that still exists).