This is true, however, many times YC doesn't invest the first time a pair of founders shows up at their doorstep. Even Drew of Dropbox didn't get into YC the first time around.
I bet the most common rejection for sensible (eg: co-founders, technical, have prototype) startup applicants to YC is "you are too determined on a terrible idea"
They accepted AirBnB thinking it was a terrible idea, and the founders would find something better.
AirBnB also were friends with the Justin.tv guys so they were already "in the mafia" a bit :). Though if they sucked I'm sure PG would have said for them to fuck off.
That said, AirBnB had a large enough vision that even if there was a 95% chance of them bombing, the 5% chance of them rocking it was too big to pass up.
Given how tight the global startup community is, I imagine a lot of YC applicants are friends with YC founders.
AirBnB sold cereal. That is determined and imaginative anyone would fund them.