Thanks for the link to Tom Preston-Werner's blog post about the founding of GitHub, it's a very good read.

He has another blog post, "Ten Lessons from GitHub’s First Year" [1]. Two of the ten points suggest that culture was important even in those early days:

  > Have Fun... Fostering a playful and creative environment 
  > is critical to maintaining both your personal health, 
  > and the health (and idea output) of the company.
  >
  > Trust your Team... In a startup, you can drastically reduce 
  > momentum by applying micromanagement, or you can boost
  > momentum by giving trust.

Although I agree completely with your point that "[i]t'd be hard to argue that the first few months of hacking was 'culture'"... It sure doesn't feel like they had to put a conscious effort into building a culture at this early stage, rather that their culture was already embodied in their personal beliefs, and the company now reflects those beliefs.

[1] http://tom.preston-werner.com/2011/03/29/ten-lessons-from-gi...

Culture is not a secret sauce added by management after the company is established. It starts with the personality and practices of the founders and then evolves and solidifies over time as the company grows. Nothing could be more relevant to the culture than those first few months as they planted the seed that grew into what we see today.