I disagree with your conclusion that it’s “the stress system”, but even if so, that’s not a mental illness then. Whatever is going on in CFS isn’t causing behavior or mood etc. changes. It’s causing physical changes, such as the measurable dysautonomia, measurable response on the 2day CPET, measurably response in the invasive CPET that Systrom has been doing, so on. Sure CFS patients do often get comorbid mental illness such as anxiety about their health and depression due to their illness, but you’ve got the direction inversed.

If you look at the evidence, all the studies so far show that CFS is triggered by stress and/or viral infections, and that the HPA axis is dysrupted. So it isn't my "conclusion", it's just what the evidence is showing.

Also, mental health is intrinsically linked to the stress system. Mental stress causes depression.

https://link.springer.com/article/10.1007/s00213-016-4218-9

If you talk to patients who have severe CFS and depression, you'll see that it isn't comorbid depression. Instead it is a fundamental symptom related to PEM. (See for example, Whitney Dafoe's description of his attacks). I had the same experience. The depression is not psychological (caused by thinking about the illness). Rather it is PEM, and comes on suddenly after exertion. That is not "comorbid".

I know the HPA axis has been an interesting line of investigation, but not everyone with CFS is showing up with HPA axis issues. I still disagree with the direction that the illness is primarily a mental illness simply because it can result in some mental symptoms. If primarily it is physical in nature, as measured on various tests, and in your case may be caused by an endocrinological dysfunction, and secondarily it causes changes in mood or behavior that doesn't make it a mental illness. (I'm familiar with PEM myself but do not have any severe mood change during it myself.)

Take for example African sleeping sickness. After initial infection the patients have a number of mood and behavior changes, excessive sleepiness, confusion, etc. that might appear mental. But it would be absurd to classify it as a mental illness. It sounds very similar to the logic people are applying to ME/CFS in trying to label it mental.

The difference is that an organic cause has been found for sleeping sickness. Also, patients don't recover after psychological therapies, as they do with CFS. It's important to be open-minded to all etiologies.

And I never said it was "primarily mental". I think it is primarily stress, which can cause mental symptoms (but not always).