I'm actually one of the people OP is referring to. So I'll do my best to answer the question from my anecdotal experience

Basically, here's how my mind works: If I end my day without having done anything productive, then I go to bed feeling crappy. However, if I do productive stuff, then I finish off my day satisfied.

If I do nothing at all, then I feel like I'm wasting my life. And when I feel like I'm wasting my life, I feel worthless. At first, it was very hard for me to do it. I had to literally force myself to get out of my bed to do what I needed to get done. Eventually, I got to a point where doing this stuff is so habitual, that I can do it without a second thought.

"How are they still moving??"

Well, this is a complicated question. Eventually, we overwork ourselves into a stage of burnout. When that happens, our entire world starts caving in on us. It's an utterly horrible feeling, but you never hear it from us. Is it because we're emotionally strong or tougher than the average person? Lol hell no. What you're seeing of us is the part of us we want you to see: we don't want people to see our insecurities, our vulnerabilities, and so on. We want to be seen as winners. So, really, we're no different from most of the people on r/depression, the only real difference is that we just don't like expressing our feelings as much and push them aside.