He was the original angel investor, first chairman, and second CEO for Apple Computer, Inc., providing critical early funding and managerial support. At the company's founding, Markkula owned 26% of Apple, equivalent to each of the shares owned by cofounders Steve Jobs and Steve Wozniak.
Additionally, depending when when incorporation happened, he may even be officially a cofounder (and not just a logical one)
Lots of people are founders in title and historical reconning. I keep seeing Aaron Schwartz listed as a founder of Reddit, and I personally agree - he was the first employee in legal fact, but he was there before there was a product, which makes him a founder in an important sense.
The company existed for 6 months before Musk became involved. It had no product, no prototype, and not even a real design for a product. It took most of a decade to turn that into a real car, all of which time Elon was involved.