You really think JeffB is walking around interviewing thousands of individual contributors and line managers, and firing the ones who are nice to each other? Senior management creates a culture for middle management. Middle management decides how to transmute that into the lower management community.

You just said it - senior managmenent creates a culture for middle management, and that culture creates the culture for the techies. If it's corrosive at the top, it tends to end up corrosive at the bottom.

If "culture" means anything, it refers to something self-sustaining. Initial conditions, which come from the founders, have a large influence. So do the values and behavior of leaders. Any development which is too deviant within an organization is eventually rejected the way an immune system rejects a foreign body. Culture is the agent in that process. That the rejection is executed by middle managers doesn't mean that they invented the culture any more than I personally manage my antibodies.