I am skeptical that you can get anywhere near 12 hours of productivity out of an engineer. Even in my 20s, I was mentally fatigued after 8 hours of (mostly) work with a few breaks sprinkled in. Once that fatigue sets in your productivity craters.
I’ve noticed people who promote these extreme work hours seem to spend a lot of time posting on (and I assume reading) social media. Perhaps they feel 12 hours is reasonable when they dedicate 4 hours to brainrot (ahem, or “building a personal brand”)