Burnout isn't an intolerance for drudgery. Everyone has that to a certain degree and sometimes that intolerance can become elevated due to overwork, boredom, stress, etc.
Burnout, in my opinion, is the loss of ability to cope even with ordinary levels of difficulty and stress in the workplace. Someone who is burnt out no longer has any degree of confidence in their ability to achieve difficult objectives. And their ability to do productive work is diminished to a tiny fraction of what they were capable of before. It's not just that they are incapable of doing things that were at the limit of their ability before, it's also that tasks which were well within their grasp are now struggles, if they are able to do them at all.
It's fear, aversion, in some cases clinical depression, and loss of confidence. In many ways it bears a lot of similarity with PTSD or shell-shock: the most serious cases of burnout involve constantly reliving past failures or stresses at work, avoidance of triggers, anhedonia, increased irritability, lack of concentration, feelings of shame and hopelessness, etc. I'm not implying that PTSD and burnout are necessarily comparable life-events per se but it seems likely they share some cognitive pathways.
That's not burnout. Real burnout is more closer to having to leave software development field for farming, because you can't function as a developer anymore.
Tbh for me the fear of burnout is enough to leave a job. I don't want to hate/ be unable to program anymore.
I describe burnout is any time an individuals focus drops to the point that their work quality potentially becomes damaging rather than adding value.