Certainly there should be no unwarranted or at least no unacknowledged assumptions - we know so little about ME/CFS that would be foolish. But if all experiences are to be acknowledged we have to use precise language to avoid conflating one idiom, label or experience with another.
Currently we have on the one hand an argument for PEM being a cardinal feature of ME/CFS together with explorations of what the pathophysiology of the (typically ?) delayed response of PEM means for ME/CFS, while on the other hand there are arguments about the utility of the term 'malaise', expansion of the chronological aspect of PEM that obviates the significance of delay, and the potential that PEM (however we are choosing to define it) is not unique to ME/CFS. Amongst all this definitional debate there's a danger PEM as meaningful term will be lost.
It seems to me that the feature of 'delay' is the most interesting aspect of PEM; if it represents evidence of a definable physiological process then that would be significant as it suggests complex biology that could be useful in unravelling any pathophysiology that is unique to ME/CFS. Symptoms that are associated with exertion without any significant delay are recognised in many medical conditions, if these non delayed symptoms are important in ME/CFS then they may need to be examined relative to known medical conditions without the need to consider unique pathophysiology.
Whatever the case if patients hope to provide useful input into research then we have to seek to maintain precision in our meaning otherwise we are offering up an undifferentiated mess.
Technically it is delay if you can complete more than you should have. I think it is easy to forget that normal people get signals from their body well before they are into a 'zone' that would lay them up for days+. Someone can't make themselves run a marathon fast enough to do enough damage, or sit concentrating on a piece of work long enough without their eyes closing to not be able to speak, read or think properly for days.
The particularly unusual thing about PEM - and that which has allowed us to be troped and called malingerers by so many who play little games - is that 'but why did you do it if it was going to hurt you' or 'you seemed to manage it OK at the time'... followed by some stupid-length payback.
Lots of other illnesses get fatigued first ie can't do in the same way or it affects performance in the first place - which is why the 'fatigue' definition was so misleading/counter-correct. Maybe that fatigue in other illnesses doesn't affect a CPET always (hence day 1-2), maybe it is an issue that does.
This flags to me how poor and problematic the whole area that calls itself fatigue research is - because it is scary it hasn't even differentiated these things (which indicates they don't intend to or see finding out what they are dealing with as a priority).
It's why analogy-wise when I describe 'tiredness' to someone who keeps using that one I say a good day for me is how they would feel if they spent 5 days in-transit on a flight with nowhere to lie down or stretch out etc. going across time zones getting up when planes land or gates are called for exertion of hauling luggage. About the only thing that comes near as it has been imposed.
By the time you get more severe your body is so fragile you are on a dental floss baseline for anything over-exerting what is a pretty wrecked body that hasn't been able to recover from substantial prior PEM. It tends not to be 'going for a walk' but 'noise' or 'moving' like you are already in a crash on top of a crash.
I don't know how you equate that with other illnesses - but actually I think that would be very insightful