Aside from some other great comments here (some of it which really
nail it) I'd like to point out that the greatest difficulty of the author seems to be being in charge of things.
Throughout the entire story he is denying control.
Being accepted into Y Combinator? Pure luck.
Most critical interview factor? Sound of biting a plastic bottle. It doesn't surprise me a tiny bit that he thought he had impostor syndrome since he probably felt he is getting rewarded for something that is seemingly totally out of his control.
Then all the "pivots". Again feeling that startup success is something that will magically come just by "waiting a bit longer" or "having a slightly better idea" (i.e. being largely out of one's control), of course you re-set the course right away if the odds seem against you. Even if it's just some objections by interviewers who know you for about 10 minutes. Because they surely know better, right? No way to to prove these guys wrong, right? Probably much better to try something different...
The author ends by saying he wants to get back into startups once he's found a suitable problem to work on. That's cool. Even better would be to find a problem he knows he can solve, by being in charge.