In the interview he explains that they did not go through a proper YC batch because "the timing didn't really work out". They had been through the process before and he had got to know pg well. Hope that helps!

I didn't think they were in one of the batches (haven't seen the usual YC'whatever badging on any of their posts' titles here). Their job post from a while back (http://news.ycombinator.com/item?id=3099362) does list YC as an investor, though.

Edit: Patrick's previous startup, Auctomatic, was in YC W'07 I think. Maybe that has something to do with it.

Listen at 13.40, he mentions that although first funding came from YC they didn't go through a YC batch - Patrick had previously been a YC member with another startup: "Auctomatic".