Being one of the people on this list... yes I would personally say it's mainly about agency and determination, and being obsessive/perfectionist about details. I think YC selects for these traits much more than domain expertise or raw intelligence.
Now that I'm working at a large org (before this my career was purely in startups), I see the importance of this all the time. When you work on a significant project in a bigger org, you get blocked by all kinds of things. Most have very reasonable explanations, but you are blocked nonetheless. Many people just accept this and allow things to proceed pretty slowly, or they accept tradeoffs that make the product worse because it's the path of least resistance. If you want to go faster and not compromise on details, you often have to be persistent and willing to follow up on things to the point of being annoying, which feels similar to being a founder.
Now that I'm working at a large org (before this my career was purely in startups), I see the importance of this all the time. When you work on a significant project in a bigger org, you get blocked by all kinds of things. Most have very reasonable explanations, but you are blocked nonetheless. Many people just accept this and allow things to proceed pretty slowly, or they accept tradeoffs that make the product worse because it's the path of least resistance. If you want to go faster and not compromise on details, you often have to be persistent and willing to follow up on things to the point of being annoying, which feels similar to being a founder.