Once a company has more than a thousand employees, it automatically becomes a company where most employees work from 9-5. Google is no exception.

You're only hearing from the 9-5ers. The rest are still at their desks working :-) It's only 7:30PM in the Bay Area.

Most HN comments are written from work.

Working consistently longer hours has been shown to be counter productive, in every study I'm aware of. This has also been born out in my role as an engineer and technical team lead managing other engineers and coders numerous times. I doubt Google needs to be informed of this!

You'll note that there's approximately zero 'goofing off' time in his schedule, he's either working productively, or not present at work. This is a good way to be productive and maintain sanity.