Amyloid/Tau are symptoms, but the issue is we don't really have a pathology or a cause that we understand. Normal brains have Amyloid build up too, I believe, but the build up clears quickly. We don't really have a clear understanding as to the functional breakdown behind Alzheimer's, although there are many theories, inculding arginine deficiency. The point is, that is a disease that is well accepted to exist, but doesn't have any pathology beyond symptoms. Chronic fatigue. At a base level, defines it's own symptoms - chronic fatigue.
And don't get me wrong, I agree with you on some levels - anyone can use logic, and it isn't sciences job to make up diseases. I think Synapsis' point is that labelling something as chronic fatigue is kind of a non-diagnosis diagnosis. It doesn't get to what is causing the issue, which is probably something that already has a diagnostic strategy. Chronic fatigue being too vague, as you say, to really be useful in treatment.
But to say it doesn't exist because of no KNOWN pathology, just isn't logical.
I'm just taking a different angle too, so that we may all learn. Greedily, I'm most interested in me learning and your challenges and hopefully Synapsin's responses/challenges will get me thinking and learning something.