This sounds like a bit of a conspiracy theory.
I think the thing that you're missing is that success breeds success. PG was able to be successful with YC because there were a few successes from the first batch, which built the reputation. That lead to more applications, which meant more people to choose from. Being smart in the choices is part of it, but there are a LOT of successful businesses that weren't part of YC, and a lot who were part of YC and not successful.
Then comes the network effect which further builds on previous successes. A YC start-up needs a hand, PG's network is best able to provide it.
The '49 equivelantly smart, articulate people who had more average outcomes' sadly, didn't have the first success, which lead to more.
This isn't only a factor with YC. Lots of YC companies have built on the success of just getting into YC. Do one thing really well, and people take notice. They offer you the opportunity to do other things really well, and your reputation grows. It's an exponential effect.