Even before taking over as President at YC, he was incredibly helpful to a bunch of YC founders and others doing startups -- a combination of willingness to help, and knowing what to do/how to do it, plus being able to do it with high leverage. Mostly weird/unpleasant situations (like the OpenAI drama was, but at a much smaller scale). Doing that for several years before YC President, then presumably continuing to do so while YC President (I'd mostly drifted away from YC by then), goes a long way to explain everything.