They needed money and Musk had it. Unless you think it is some kind of one-in-a-gazillion opportunity where an EV enthusiast also has millions to chuck around? The vision for Tesla was already mature enough that capital was provided.
"The vision" you're just full of BS, you got your previous claim completely backwards as you saw for yourself from the source you demanded now you're coming out with some claim about a "vision", they all had a general vision to form an EV company, that's why they teamed up.
Musk wasn't a genuine founder.
Yes, he was. That a company had been incorporated already is completely moot here, you should know better than that tbh.
Musk took a ride in a tZero after JB Straubel told him about it, when Elon learned that the guy who built the tZero wasn't going to produce it commercially he asked if he minded if he tried to do so... and that's when he was put in contact with the guys at "SRA" who were looking to do the same... ergo they teamed up.
That "SRA" was already registered as a company with that "vision" (as dlockers puts it) is kinda moot, here we have those two (then three) individuals and their company set up ready to pursue the same goal as Musk and Straubel so they came together to pursue that goal and all five of them are considered to be co-founders.
If Elon had registered a new company "Tesla" with Straubel and asked the other three from "SRA" to join that instead would that make a huge difference? It's a technicality, five guys got together and founded a car company, there wasn't some existing product or IP he's buying into there, they didn't even have the name Tesla.
Nor was he some passive early investor, he wasn't a full-time employee initially but he had significant input into the very first product they produced; the roadster.