I think they have. They've indicates this in their updates of their CBT-model.
Knoop et al. 2010. The central role of cognitive processes in the perpetuation of chronic fatigue syndrome.
“There is less support from treatment studies for the role of the other perpetuating factors that have been postulated in the models of CFS, like a reduced level of physical activity, deconditioning, and the tendency to attribute the illness to a physical cause. The first two did not mediate the treatment effect and the last one did not predict the response to CBT and was not changed after CBT.”
Wiborg et al. 2012. Towards an evidence-based treatment model for cognitive behavioral interventions focusing on chronic fatigue syndrome.
“A study by Deale et al. showed that somatic attributions of CFS patients are not affected by cognitive behavior therapy (CBT). Without such a change, a factor cannot account for the effect of treatment. This finding is also in accordance with the model of perpetuating factors in which causal attributions are not directly linked to fatigue or impairment [11]. Consequently, we excluded this variable from further examination.”
Heins et al. 2013. The process of cognitive behaviour therapy for chronic fatigue syndrome: Which changes in perpetuating cognitions and behaviour are related to a reduction in fatigue?
“somatic attributions were not analysed, as previous research had shown that somatic attributions do not change during treatment, so they could never be a mediator of CBT.”
In their updated model it's no longer about reversing deconditioning, increasing activity levels or changing somatic attributions. It's mostly about focusing less on symptoms and self-efficacy (basically the placebo respons).