I'm guessing that YC's existence as a seed fund has a lot to do with it. How do you sell a company that is essentially a one-man shop? If I were buying a company, I'd want to see that it's leader(s) are team players.
As the article says, you can draw other people to the company as soon as you can afford it. There you can demonstrate that you are a team player (and a team leader).
It's not that hard when you're selling a company with cashflow and / or IP. I don't think being a single founder and being a team player are necessarily orthogonal.
