Do academics actually work "full-time"?
Full time is a misleading term. Postdocs, I suppose, normally have supervisors who set the conditions for work. Something like what you've experienced. You meet those expectations or you have problems. So yes, such people normally work full time by the supervisor's definition of it. It may include scheduled hours or not. Lab work often needs to be scheduled, but some lab work needs to go on day and night. 

Professors, on the other hand, especially tenured professors, normally don't have supervisors in the same sense. They set their own conditions of work. They set a schedule and keep to it sufficiently well that they can be productive. But the schedule might be erratic. 

I've known professors who are able to take off a couple of hours in the middle of the day to take a bike ride. But they may also work after supper or on week ends. I doubt that many of them keep a listing of total hours worked. I never did. 

In fact, intellectual work is better driven by ideas than by a clock. If I'm building something important, the clock may completely abandon me and I'll work until I am exhausted or the idea flow stops. If I don't have any interesting ideas, then sitting at a desk or computer has no value whatever. I'm better off going off on a walk or something relaxing. That doesn't mean that my brain shuts off. I just stop trying to force it to work when it won't. 

Time is a terrible measure of productivity for intellectual work. There is no assembly line for good research ideas, though proving it out can be a bit more methodical, even tedious. But that's why post docs work a fixed schedule.