Freedom would be much easier to gain if you go in with a reputation. In comparison, a startup probably offers a lot more freedom to someone smart, but lacking the credentialing of a previous start-up success. And anyways, freedom for its own sake is not really useful. Some ideas are complex enough to require a team of talented individuals and perhaps more resources than what a startup can muster in its initial stages. Google probably has a few great and interesting projects that he interested in, and the lost opportunity of making millions through a sale or an IPO might matter less to someone who has already trekked that trail.