Well GET derives from a model where CFS completely psychiatric (buzzword is actually psychosocial - you're behaving wrong, doncha know).
In this model, there is no organic illness, and you can exercise without coming to harm. The idea is to put you on a low level of exercise that doesn't make you too tired/symptomatic, and then crank it up saying, "See, that wasn't so bad was it," until you are physically much fitter and stop having these faulty beliefs that you're ill, because you're clearly not.
The motto of GET type treatments I was given was "Don't follow the symptoms, follow the plan." Because the symptoms are all mistaken psychosomatic manifestations, and not a sign of physical harm in progress.
If a medical team believe in this model, POTS is a bit of showstopper. It's much harder to write off as non-organic, and it clearly does involve the heart responding drastically to exercise and posture. So I can entirely see that a team which thinks GET is a good idea would claim you can't possibly have CFS and POTS: it undermines their model if CFS-sufferers have organic illness.
If on the other hand you have a medical team who think CFS is something they don't understand, but here are some diagnostic criteria (based on exclusion, not positive identification). And POTS is something of which we don't know the cause but which has spedific diagnostic criteria. Then the medical team will be comfortable that the two can easily co-exist. Or one can even say that POTS is one of many conditions under the umbrella of CFS, which after all was only a dustbin diagnosis in the first place.
Well that's my understanding, anyway. Sorry, hope my cynicism doesn't damage anyone's hope, if that's what's keeping them going.