Only 25% of sufferers are house or bedbound and I agree that their illness will be exacerbated by inactivity, but for the rest it is different.
To overstate it a bit, if you have a very inactive lifestyle you are unlikely to get PEM and that does not fit with most people with ME who continually do more than they should. During the epidemics, it was the active people who became ill. One was in a teacher training college set in a convent. The students became ill, the contemplative nuns did not. There is a certain truth to the idea that people who get ME are the ones who push themselves.
My experience of moderate ME may not have been typical of sufferers now as we did not know so much then but I found it much easier to walk than to make the complicated actions to use public transport and could not drive a car because of sudden onsets of paralysis. I did much more than the government guidelines. Friends with ME at the time also lived an active enough lifestyle in that they worked but spent the evenings and weekends resting.
I seem to remember reading somewhere that people with ME do not show the muscle wasting that would be expected - it might have been during the outrage over the DWP description that severe ME could be identified by muscle wasting since it is so uncommon. Fitness in the muscles comes because damaged mitochondria give a signal when there has been too long spent in anaerobic respiration (the burn). It is possible that this signal is given in ME so we are fitter than might be expected.