I'm not sure if we know that for certain. The 2xCPET studies suggest that PWME's anaerobic thresholds decrease after we have been active.Presumably PWME get PEM well below anaerobic threshold because going to anaerobic threshold is not something anyone does unless they are training in a gym, but PWME get PEM during daily tasks.
So, on a good day I can hang the washing out without problem, while on a bad day, that activity may perhaps cause me to exceed my anaerobic threshold (my heart rate would suggest that at least). And my 2xCPET results showed a big drop in anaerobic threshold after exercise, even though I did not feel that I had PEM.
I can see some utility in this kind of research. It would be useful to know if PEM only occurs if the anaerobic threshold is exceeded (or 60% of anaerobic threshold or whatever). I participated in an earlier study by this team. There did seem to be a belief that exercise of the right sort might help to move us towards health, but they also did seem to be keen to do good research.
I do have concerns and yes, we certainly need more detail before we could be happy about the research. For example, when I did the 2xCPET with them, there were inadequate controls around non-test related exertion before, after and during the test. Some people travelled a long way to take part in the study. They will need to be clear with participants about what non-test activity may be done and really they should be using activity monitors to ensure that their requirements are followed.