You can work 20 hours a day, but to do so consistently and without weekends is insane.
"Top execs" work a lot, sure, but they too are human and need sleep. 20hrs/day 365days/yr is unsustainable and I doubt it's even possible.
Maybe the issue is that I'm thinking "billable hours" (that is, hours you are actually working and could bill a client for them), whereas they could simply mean "hours on the job-place" (that is, hours you spend sitting in front of a computer or in an office), where you'd get something like 30~60% productivity. If you discount lunch breaks and commute time, and you work from home, the hours become intertwined with "waking hours", which are suddenly a lot more believable.