What's the CEOs fiduciary duty again?
CEO answer to the board, but a board can be controlled by the founder in one way or another. For example, Zuck control shares with more votes and Bezos have vote rights over his ex-wife shares. The CEO can also fill the board with loyalists, as Jobs did on his return.
He sued the original founders into granting him the title. Although it really shouldn't be that big a deal he's labeled as founder. When you're the CEO you can just generate titles anyways so he could be called the The Excellent Supreme Leader Always (TESLA) and it'd be allowed.
In layman terms he's not a founder of Tesla but titles aren't required to fit a layman's definition.
https://ir.tesla.com/corporate/elon-musk
> Elon is Technoking of Tesla