According to Newell[1], Valve aggressively fires as it hires. It's not based on stack-ranking bullshit - more of a cultural and disciplinary fit. If you have the right personality you could thrive there, but if you need a manager to help you shine it's best to look elsewhere.
It's based on capricious and cliquish factors that aren't documented anywhere. Having the "right personality" to thrive there means knowing who to ingratiate yourself with.
Source: close friend is an ex-valver.
That Valve Employee Handbook is the 2012 edition. Is there a newer publication?
I think GP meant the Valve "New Employee" Handbook
The "culture" part might be good, but the end-result is Steam-- lousy software which has been lousy for decades at this point, and is unlikely to ever improve.