Let’s be honest, if you ever worked in East Asian countries with these work ethic culture, you’d know they just sit around the office doing absolutely nothing. It’s bullshit.
I love nonstop work, the problem is that a lot of times I have to unblock problems, stop to talk to other people, there are dependencies or things are on fire so a lot of time is spent busy waiting. It is work but it is not rewarding and you don't actually get much done.
Yet, if people were only working 16 hour per week, the company would stop to a crawl. Because during my extremely productive, I might need a few non productive hours from you. Maybe it's answering a quick question, maybe is giving me a pointer to something, on inviting me to a document, enough that I can keep progressing without being blocked and getting those golden 16 hours interrupted and wasted.
The point is that, even tho these are not productive hours for you, they are creating a massive value for the company because you can be supporting other people productive hours. And maybe those are happening on a Saturday from 6 to 9 PM. In fact it might even be OK for the company to tire you a bit, to get that availability, even if that means you won't have 16 hours of quality work anymore.
This is not in defense on the 996, but it is to balance the whole point that people should only be working when they really are productive/focused/.... Availability, even non productive does matter as well.