From what I've read, he bought in as an investor and forced the actual founders out.
He could just as well have founded his own company, then he'd be a founder, because that's what that word means. I don't see how you should be called a founder if you aren't, well, a founder.
Not that I see anything inherently bad or unimpressive with not being a founder.
The rest of your comment neither refutes my point nor reinforces the earlier comment that Tesla was nothing but a name, brand and prototype: Chairmen oversee board meetings, not gnarly technical challenges on the production floors.
No doubt once he became CEO (after the roadster was shipping) he became more involved in the day to day.
He could just as well have founded his own company, then he'd be a founder, because that's what that word means. I don't see how you should be called a founder if you aren't, well, a founder.
Not that I see anything inherently bad or unimpressive with not being a founder.