This is similar to the idea that it doesn't matter where you go to college - because the things that get you into top X schools are the same traits that correlate with success afterward. This paper is a study that makes that argument: https://www.nber.org/papers/w7322 (disclaimer - I only read the abstract)

Also - in fairness to YC - there are a staggering number of companies that are started every year. I don't have the stats but I would guess that YC outperforms the totality of the startup market.