I wonder if I'm just a lousy human being, or are these startups full of cheats. For me, working 25-30 hours a week (really working, not pretending to work or sitting disengaged in meetings) is probably a long-term maximum of sustainable effort. How the hell are these people working 80 hours a week?

They're not. I've done maybe a dozen genuine 80 hour workweeks in my 30 years since high school. Maybe. At least half of those were in college.

People who claim to be regularly working 80 hours a week are often over-counting ("I can't remember doing anything but sleeping and working last Monday, so I must have been working 16 hours.") and often not actually working during 30-75% of the time they're ostensibly at work. Walk by the random colleague at a random time and note how much time their screen has code on it vs Facebook, YouTube, or an online shopping site. Sure, they might be studying an online course on YouTube or buying a critical piece of lab equipment for work, but more than likely they are just dicking off about half the time.

in my experience a lot of the younger people spend a lot of the day at work messing around. (socializing etc). they also come in for breakfast and dinner and go to the gym. and then stay late working since they didn’t get much work done during the day. If you naively count the hours spent at the office it does indeed add up to 80

They aren't, or if they are there's probably drugs involved. In the 80s, it was cocaine.

Now it's adderall.

I prototyped something for a startup I was working on, and put in about 80-100 hours a week for three months. That was probably the most I could have managed. I went a bit crazy. My dreams got weird.