Firstly, everyone had free, good quality, sound blocking headphones. Not just free as in standard equipment on your first day free. There were buckets of the things lying around and if you needed a new set you just grabbed some, no questions asked. VC funded opulence, yay!
Secondly, there was an understanding amongst everyone on the shop floor that “headphones on” meant do not disturb, just as much as a closed office door. Violations were not tolerated but it was a social norm thing not an HR write up thing. Maybe people were getting the latter behind the scenes though?
In return you end up with an office architecture that’s considerably easier to manage at the expense of turning the physical challenge of giving everyone an office into a — potentially intractable for some teams — people management challenge.