I think you are absolutely right to suspicious of such narratives. They are always a product of both the observer's bias and the systemic bias of the environment in which they operate. I didn't immigrate here, which changes how I experience the place, and as someone who tends to work for 5 to 10 years at a place the half dozen places I have direct experience at are a small fraction of the total number of companies.

The effect I've observed is that Silicon Valley has such mobility (which is to say between jobs) and such commonality between business structure, that the larger companies get, the less pronounced their differences become. I attribute that to the challenge of assimilating folks into the company culture that is too far off the valley 'mainstream.' It also provides a tremendous amount of cross fertilization where people say "Oh at my old company we used to do X" and if its a better thing than what the locals do it has a good chance of being adopted. So management techniques, office layouts, and programming methodologies have a sort of viral nature to them as well. This seems to homogenize the way companies evolve than you might expect.