I can understand that being the case for some people. But that isn't the kind of studio Valve runs, people work in very small pods or just entirely independently there from my understanding. The issue of motivating yourself in their case is always true, so I have to assume that is factored into their normal productivity. Working from home doesn't change much where they work slower because they can't collaborate or communicate if that is minimal to start with.
Its especially odd that their productivity drop is higher than studios that do rely on collaborative environments and teamwork more.