One of the things you must realize to make sense of a lot of PG's advice is that the default business model for YC companies is to build a promising product so that venture capitalists will give you money or that a Yahoo/Google/Microsoft will buy your company.

For those two goals, which are based as much on impressing investors as on anything else, your chances are much better if you're not a single founder. Investors are much happier investing in a team than they are investing in a single person. To that end, PG's advice against single founders is spot on.