Doesn't really matter the type of doctor, spending all their time on revenue-generating activities would seem to be better than only 75% generating revenue and 25% on "administrative and bureaucratic tasks" that don't generate revenue and which could be accomplished by a much lower-paid employee ("secretaries and assistants").

Perhaps you're correct that the doctors are simply working much longer hours but that's one group of employees among a hospital's staff who do generally have a lot of power and aren't too easy to make extraordinary demands of.

There are reasons why the claim might be right, as noted by others, and there are reasons why the claim may not be right, as noted by you. If you think that your idea of how Doctors operate in a hospital is more compelling than other people's explanations of why the claim is legitimate, then keep believing that.