He had an older brother, who was, by the younger brother's own admission, a notch above himself in the intelligence department. The older brother was said to have been bored working at Google.
My suspicion is: someone in dire need of intellectual engagement might want to work at Tesla simply because of the opportunity to exercise one's mental capacity in a challenge of equivalent magnitude. (Not Tesla specifically: I suspect any company with similarly-driven heavily-pushed technical production would suffice, for a person with the knowledge of that sphere.)
Imagine Einstein or Hawking sorting paper in an office. How do you think their minds would feel in such an uncreative, unproductive environment? I'd wager they'd suffocate soon without an opportunity to express their intellect in a meaningful way. Same story, I suspect, happens with the brilliant engineers and designers at Tesla.
(Which isn't to say that the nightmare-ish conditions they're being put through are necessarily worth it, or that they can't find an outlet for their capacities elsewhere. Working for a company is simpler, in that one doesn't need to make as many choices as an independent entrepreneur would have to. It's also prestigious – since the company has a big name – which may or may not play its role.)