"Zero passion 10am-4pm" doesn't necessarily describe not working hard. You can only do three to five hours of sustainable deep work a day unless you're a freak of nature. Usually anyone saying they do more just pads it out with breaks on Hacker News or Facebook, or has failed to automate the repetitive work in their job. A focused, organized engineer can pretty well exhaust their sustainable capacity plus do their overhead in 10am-4pm. As for zero passion, why do you need passion? And what does it have to do with not slacking?

Facebook makes you demonstrate your impact to the company twice a year. You would be hard pressed to be a slacker long term there.

The work does appear different because at that scale you spend a lot of time tracing out what's there, figuring out context, and trying to figure out how to make a change without bringing down everything else. If you're used to a small company or a small codebase where you spend most of your time writing new code it seems less productive, but it's mostly just different.