While Tesla existed before Musk joined it, it also had zero sales — Musk was part of the series A funding in 2004, their first public prototype was 2006, Eberhard was asked to step down in 2007 and did so Jan 2008, the first actual delivery Feb 2008 — and also electric cars were a joke at the point Musk invested in Tesla (the hype was hydrogen and ethanol to be the green fuels of the future). As 99.7% of the market cap gain happened since 2010, and therefore under Musk’s leadership, I definitely count Musk as more relevant than Eberhard (although even then, organisation and salesmanship is not enough by itself, but I’m not going to get sidetracked here into means of production).

SpaceX, however, I believe was literally founded by Musk.

That is poor use of rhetorics.

Because he didn't start it doesn't mean he didn't contribute to the development of the company.

Also, no one can claim that success would have been at the doors without his input.

Note: I'm not defending Musk here as much as I am opposing the rationale of the argument.

Pretty sure Elon founded SpaceX himself. If you have a good source for the claim that it existed before he came along, you should take the matter up with Wikipedia.

SpaceX existed before he got there? Citation needed.