My understanding of the story was that YC was going to pass on Airbnb but then one of the cofounders mentioned the cereal box story and that changed PG's mind
YC also despised the idea, but invested because they liked the founders. But they also presumably liked the 600+ YC founders that failed, so you can see how much investors can really predict.
Even to this day I'm astonished that AirBnB exists and yet YC was able to see something in them years ago.