Even though it's hard to find any flaw in "Use the right tool for the job", I still think that the success of a start up in Silicon Valley (and probably the rest of the world) has a lot less to do with the technical choices of the company than the people who work there. And luck. A lot of luck.
It's still baffling to me that pg doesn't realize how lucky he was. He was in the right place at the right time and he was probably surrounded by competent technical and non technical people. If you have such a team, the technical preferences of the engineers will probably have very little bearing on the success of the company as long as they use the tools they are proficient with and which allow them to execute quickly.
I mean, the fact that Rails and PHP were at the origin of Twitter and Facebook alone shows that the quality of programming languages is irrelevant for the early year(s) of a start up.