I can understand a little bit this mentality if you're working for yourself as a one person start up with your runway quickly drying up but to sacrifice your life for a soulless corporation who couldn't care less if you died the next day is very puzzling.

I wrote that as someone who is currently bootstrapping a company while working a part-time job alongside.

In the situation of a single person startup then you need to be smart about where to put the time and effort, rather than burning yourself out by overworking.

Since who will sustain the effort in the long run if you wont be able to?

I strongly agree. If you're in a limited resource/time scenario it's not about doing more but ruthless prioritization. You will not do very important things; make it intentional.

Very true and Carmack very much was working on his own startup on a passion project

But they are technically slacking off, aren't they?

It doesn't matter what the reason is, but you're not getting anything done in that time. Why should you consider that as a time you worked during?