Yes, burnout is not really about how much you work at all. It's caused by a mismatch between your values and your work. For many people here their values probably include curiousity and wanting to learn. I burnt out almost completely doing work that was easy - but that I totally and utterly despised because of how boring it was (and how little it would advance my career). This was indeed like learned helplessness (not being prepared to quit immediately when I had the means - which was actually at any point (!) - was the learned helplessness part).
In other words, you can burn out at a job that's 12 hours a week and not burn out at a job that's 60 hours a week. It depends heavily on the nature of the work. Whenever people talk about feeling 'burnt out' purely due to how many hours they work I am always sceptical and suspect the root cause is actually different (note that I actually think our work week is too long even at 40 hours, I think too many hours of work are unhealthy for other reasons). In this sense I'd distinguish 'exhaustion' from 'burnout'. The former can turn into the latter but often does not but the root cause of exhaustion is exertion whereas this is not the case for burnout. They can converge, of course, the ultimate result may be severe depression in either case, but it's still worth distinguishing.
In other words, you can burn out at a job that's 12 hours a week and not burn out at a job that's 60 hours a week. It depends heavily on the nature of the work. Whenever people talk about feeling 'burnt out' purely due to how many hours they work I am always sceptical and suspect the root cause is actually different (note that I actually think our work week is too long even at 40 hours, I think too many hours of work are unhealthy for other reasons). In this sense I'd distinguish 'exhaustion' from 'burnout'. The former can turn into the latter but often does not but the root cause of exhaustion is exertion whereas this is not the case for burnout. They can converge, of course, the ultimate result may be severe depression in either case, but it's still worth distinguishing.