Exactly. The list consists of the successful startups that they founded, but not the ones that they tried and failed at before finding success.
I'd like to think that most successful founders fail at their first few not so noteworthy "side-projects" before finding success and sticking to it. Just building a crappy version of something not-so-cool can teach you a lot about tech, business and design, which then helps you build better products in the future.