While certainly some with ME/CFS are so bad that they cannot get out of bed the vast majority have good and bad days and you don't see them on their bad days and they look just like everyone else on their good days. One of the reasons it's considered an invisible disease. It's a spectrum and some people manage it better than others.
Sometimes the burnout isn't so temporary - even this article talks about how repeated burnout becomes an problem with the person entering a state of depression which bares a lot of similarities to ME/CFS.
Most people with ME/CFS will never be diagnosed and those people will often think they just have issues with burnout. That's what I though for over 20 years and I have a severe form of it. I though I was managing burnout and then graves disease. It never occurred to me that I would have something like ME/CFS until I stumbled on a random twitter comment that linked ME/CFS with hypermobility which I also have a severe form of. So here I am adding my comments to the internet.
There are generally a whole bunch of rather weird and rather rare comorbidities that occur in people i.e. it is known by its associates. That's what a lot of Dr. Jessica Eccles work goes into.