As he said in this tweet, even with 8 hours of sleep, there's a lot of time left per week and people like Carmack makes the most out of those remaining hours.
I know a lot of people outside the normal 9-5 job who overwork themselves and takes less than 8 hours of sleep though. I think it has a lot to do with the fact that people just react to stuff and it's easy to come up with short term thought process of "less sleep = more work", instead of working fewer hours one day in order to keep long focussed hours in the long term. Also, some people can't afford 8 hours of sleep, as far as I have seen, unfortunately.
As he said in this tweet, even with 8 hours of sleep, there's a lot of time left per week and people like Carmack makes the most out of those remaining hours.