It's from Valve's site: https://www.valvesoftware.com/en/publications
I also know it's real because ex-employees have complained that the handbook is full of lies. It's just meant to entice people into joining Valve. Ex-employees said Valve is not flat but run by a board of directors, you're pressured to join projects and threatened with being fired if you don't, and peer reviews are terrible because your peers use them to make you look bad so they look good by comparison.