To be fair, Woz wasn't just a “friendly selfless genius inventor engineer”, he was also the co-founder of one of the most valuable tech companies in the world. And YC is, in their own words: "The Y combinator is one of the coolest ideas in computer science. It's also a metaphor for what we do. It's a program that runs programs; we're a company that helps start companies.". They're not entirely unrelated.

He was a cofounder because of his skill and Jobs talking him into it. Woz would have been perfectly happy as an engineer at HP, that was his plan.