I'm curious: how did you extrapolate from Wil's points on how to be a successful YC applicant that he also thought all YC companies are going to be successful?

These are qualities for successful founders, not successful companies. There are a myriad of reasons why a startup might fail. Having founders with the traits outlined in this essay helps prevent the founders from being one of those reasons. That's why YC looks for these traits.

Since YC screens applicants, they can select the ones they feel are best suited for success. Based on past experience, those with the traits in this essay are the most successful. Minimizing the founders as the source of failure means YC might have more successes, on average.

As pg said, even YC would be surprised if their "success" rate is more than 50%. Being accepted into YC doesn't guarantee success... Creating a successful company is hard!

I don't really think that the post implies that belief at all. This line in particular demonstrates the number of people already IN YC who are still rather unsure about things: "one of us remarked that looking at everyone else in the batch, they're still in disbelief that they got in at all, and a bunch of us starred the comment in agreement, even those batch mates that I think are amazing"

I think that for the average startup your chance of success would improve in a statistically significant way but this would be really hard to quantify as if you get into YC you were probably already more likely to succeed on your own anyway.

It also depends a lot on how success is defined, in the return on investment for investors or in how it effects/improves the founders/employees lives. Getting a talent acquisition and working on something awesome at Facebook or similar could be a big success for the founders even though there is little there for the investors.