In other words, the warm encouraging phrases have qualifiers - you have to figure out what they are and if they apply to you.
Like this essay, the book gives much advice of the "don't give up" type. But the book also mentions some cases when you should quit: you should quit activities that are a cul-de-sac, those in which you are not committed to become the best, or that don't help you become the best at what you most want to do. The rationale is that current society rewards the very best, ignoring the rest, out of proportion with actual quality/merit. I'm oversimplifying of course.
Obviously, PG doesn't think any of the YC startups is in a cul-de-sac, so he can dispense with that exception and tell all of them to just push forward. If anything, he warns the folks not to 'unquit' school and other attention/commitment escape valves.
[1] Edit: I was focusing on the "don't give up, don't lose focus" part. This essay has other big theme, shame as a motivator, which is not touched in The Dip.
Otherwise, the qualifiers that apply to any other startup in general, apply.
you dont have to get into YC to succeed, but understand he was talking at an exclusive event - the final YC dinner.
Also understand the odds. He picks teams most likely to succeed. If he rates odds of success as 1/2 (or whatever the article said), and the odds of getting in are 20/400, then the odds of success for you and me, those who didnt get in, is 1/40. Which is 2.5% and vibes with I recall that people generally say that startup odds are 2%.