When I read ''9 to 5'' in this context I take it to represent a culture and mindset of limited engagement and responsibility, not necessarily the literal physical presence for only those hours of the day.

I don't get this attitude. It's decently well known that there are diminishing returns once you pass a 6-8 hour workday. Why would anyone want to spend more time working to produce worse results?

You may be placing a different interpretation on here that the author intends. . Here "9-5" means people who are just working for the sake of working vs startups where there's a tangible goal to hit and people want to hit it and move on. So it's a "I want to work around a constant short term goal oriented culture vs one where goals are measured in years (if at all)"

I'm strongly in your camp. However, to play devils advocate, a founder has to make the company the focus of their life. They're gonna be working long hours, 6-7 days a week, with little to no vacation. There's diminishing returns on that time, but the pressure is on and the clock is ticking.

Naturally, founders want to hire people who also make the startup their life. If you're working all day, don't you want to build your company with people who do the same?

But besides founders fervor to work round the clock, some people are just addicted to work. They can't stop thinking about it and no matter what they're doing, if it's not work, they're not satisfied. These people want to work with other people with the same mentality.

According to his linkedin profile he spent 4 years at Google in Zurich, 1 year at Microsoft and less than a year at Facebook:

www.linkedin.com/in/dimakorolev

He doesn't state whether he worked at Facebook's Menlo Park or Seattle office.