My impressions are made from afar, but it is that YC is a high intensity immersion/sprint for founders to catapult themselves to the next level while having the support of experienced advisors and colleagues in the same situation around them. In other words, help shape and catalyze founders to become great with anything they touch (and recognize what to avoid). Strictly in the context of YC (or TechStars, Excelerate, etc.) it seems like that becomes lost when working with founders at a higher experience level.