No, it's not a matter of severity. In short, there's more to ME than fatigue. Or a headache. Or a sore throat. Or all the other common symptoms in the CDC criteria. A good scientist nitpicks, to be accurate. Fatigue is subjective. That is perhaps why Powell et al managed to reduce fatigue after CBT lasting three hours. Lots of people suffer form fatigue, headaches, sleep disturbances and sore throats. As you get older, the memory goes, especially if you're also menopausal. So as researchers have already pointed out, you need to get away from the CDC criteria, ergo, the alternatives. The latest are all tending towards the old concept of ME.
Is CFS the same as ME/CFS or ME? Well, ME can occur in epidemics (name me an epidemic of CFS - bit difficult when you can't diagnose it for six months), and is closely linked to enteroviruses. It is also associated with Summer bugs. Can you say the same about CFS, or ME/CFS? At the moment, following the evidence, they are different. They overlap, but are different and that is what Ramsay thought too. And a lot of other doctors.
ME is an acute illness, though has a tendency to become chronic as Ramsay discovered around 1978. (That is his claim to fame. He didn't think up the name but he was the one who showed it tended to be chronic). There are mild forms. People can work, and do. In my studies, I had a few who worked fulltime. It depends on the symptoms and if you can compensate for them. But, as you should know, fatigue is NOT the core symptom of ME. It's less common than CFS (1 per 1000). I've summarised it all in the criteria paper. They are different unless you're a CBT fan who will tell you, they are not. They lump. It's all stress and fatigue.
If you don't define your research sample well, you can introduce confounders and make the results impossible to interpret. Another example, GET works well if you take the course at Barts, but it's not as effective in the USA. Different concept of CFS and ME. In the US, CFS is closer to ME than in the UK. But research HAS to exclude people with chronic stress or you get results like Heim et al etc.