There are multiple definitions which do not require it e.g. Ramsay ME, London ME, Jason et al's revision of the CCC, and the ME-ICC. All these definitions do require fatiguability and/or other post-exertional symptoms. Fatigue and fatiguability are not the same symptom. PENE and fatiguability are not synonymous either, but PENE does include fatiguability.
As others have said, the fatigue experienced is different than ordinary fatigue, and I think research by Jason's group confirms that fatigue is multi-dimensional and the experience is clearly different between patients when fatigued vs healthy controls when fatigued (same goes for what healthy people identify as "PEM"). Reducing "fatigue" levels to a single number may be misleading. Many patients feel that "fatigue" does not adequately describe what they experience, and try to distance themselves from it and the chronic fatigue shambles. Others may argue that exhaustion is just a synonym for fatigue.
As others have said, ME is much more than the sensation of "fatigue". Many patients report that even they do experience significant fatigue, it is not that (baseline) fatigue which particularly bothers them that much or stops them from doing more activities, it is all the other symptoms such as poor stamina, pathologically slow recovery period, post-exertional symptoms, neurocognitive impairment, orthostatic intolerance, flu-like symptoms, pain, etc.
Some patients also report that if they are rested enough and do not overexert themselves, they do not experience any "fatigue". I think it is relatively rare that a significantly affected patient does not experience any fatigue, mostly because the more severe one becomes, the more difficult it is for the body to sustain the activities of ordinary life. So even if they do not experience baseline fatigue, merely being awake is causing post-exertional symptoms which may include exhaustion. I suspect severely affected patients, the ones which experience PENE anyway*, are in a constant state of PENE.
* A few patients seem to be bedridden by severe post-viral fatigue without other major symptoms or even obvious PENE? I met someone like that once and she recovered in a few years.