I prefer the term post-viral fatigue too. However it’s incorrect to say that we don’t know if it’s a psychological or behavioural issue. GET and CBT have been thoroughly rubbished as interventions. Not only are they ineffective but they are dangerous.
I get to hear the leading experts* once per year talk about this and they have absolutely honed in on immune response and mitochondrial dysfunction as most probable causes. They are at the stage of proposing diagnostic criteria now. Things could get worse before they get better but we can confidently say that this is a medical condition now.
I’m thinking of Dr. William Weir and Dr. Nigel Speight, among others.