Interesting that it doesn't mention the craigslist spamming. I'd wager that's far more responsible than YC. As great as YC is for many things it's not a significant source of traction for most startups.
To be honest, while reading that line, I thought it was slightly odd because it was pretty specific (e.g. "outside" could be removed, but it clearly distances Airbnb from said sales people) and didn't necessarily bring anything to the story, especially since Airbnb has said the efforts were "largely ineffective".
The cynic in me feels that it was included specifically to add weight to the company's response about that craigslist story.