I think quite simply in terms of the numbers. YC results in a lot of successful startups. YC won’t guarantee your success, but statistically speaking, your chances are much higher.
I'm willing to meet you half way. The stat is misleading because it implies causation. But that implication makes it no less accurate.
It could be that being accepted to YC is just a good indicator that a company already had the necessary faculties to become successful. Even if that's the case, I imagine a control group of companies who were accepted to YC but chose not to attend would have a lower probability of being successful.