On the contrary, I don't think there is much evidence at all that the Mark Zuckerberg's of the world have so much innate talent that circumstances don't matter. The more we learn about what explains individual success, it's clear that there are lots of factors pushing a given person towards being successful, and then a lot of other factors pushing that person toward being unsuccessful. There is no "best and brightest," there is a murky combination of talent, privilege, environment, education and luck.
It's impossible to know if Mark Zuckerberg would be a billionaire if not for Harvard, for both easy reasons (maybe if he went to Yale, he would have slipped on a banana peel and then gotten hit by a bus) and deeper ones (how much did luck play a role in Zuckerberg's success, and did the network and prestige of Harvard help him capitalize on his good luck?)
Zuckerberg had many more factors in his favor than just Harvard, that's for sure. Anyone who tries to explain his success by saying "oh, that's because he went to Harvard," is wrong. But it's also impossible and silly to claim that we can know that he is just so talented and intelligent that all contingency removed he still would have become a billionaire.
This applies to the broader argument about Y Combinator. YC has a competitive advantage in that it attracts companies that have many positive factors behind them to apply. But that doesn't mitigate the possibility that YC itself is another positive factor, or that the only benefit to participating in YC is knowledge. From the outside, it seems like the applied knowledge expressed as case-specific advice and mentorship would be hard to "democratize," and that the intense amount of mutual support (start ups patronizing each other and becoming an instant, deep professional network for each other) are also not really expressible as knowledge.
Access is an incredibly powerful resource that YC can provide its companies, and one that has absolutely no correlation to being "the best or the brightest." The reason there are so many successful white male founders is not because they are the "best and the brightest," but rather because they are culturally and ethnically connected to networks that provide them with access to business opportunities and conversations and platforms and education that aren't nearly as available to non-white non-men yet. There are countless "best and brightest" women of color who do not have all of the favorable circumstantial factors that made success easier for Mark Zuckerberg (and obviously made it easier for him to get into Harvard in the first place in various ways) and yet are equally brilliant and talented.