Earlier quoted context omitted.
Most people do not really work 70 hours / week. They just are present at work.
You assume that workload is binary you either work or you are just present. The intensity fluctuates even if you study for 1 hour. This isn't something exclusive to those who work more.
For many jobs being available to do work has value, but a doctor on call that’s sleeping in the hospital isn’t actually working. As such it’s a perfectly reasonable to separate working from bing available to do work.