I understand the proposed mechanisms of impaired aerobic capacity in the "Low, Low-Normal, and High-flow" groups and I don't doubt that dysautonomia plays a role in compromised blood flow in ME/CFS, but it seems the authors were more broadly applying proposed small-fiber neuropathy mechanisms than warranted by the 31% who had it.
Yes, in my opinion the article is poorly written and appears to implicate the small fiber neuropathy as the CAUSE of the dysautonomia, which, while plausible, is speculative.
It seems much more likely to me that the small fiber neuropathy is a RESULT of the dysautonomia, not a cause. That is, the loss of blood flow to the capillary beds leads to the death of the small nerve fibers. This interpretation would be better supported by their skin biopsy findings.
I'm also not clear how they ruled out mitochondrial dysfunction as a possible mechanism for the low oxygen extraction (@Pyrrhus do you understand this).
I'm not sure that they ruled it out. From my reading, they acknowledged mitochondrial dysfunction as a possible contributing factor to the impaired oxygen extraction. They simply said that their methodology could not support or rule out this possibility.
One of the big problems in their methodology is their use of a convenience sample in the patient selection process, which probably lead to a selection bias or two.
So perhaps the "Low and Low-Normal Flow Groups" also have difficulty extracting oxygen, but visibility to this is occluded by the greater oxygen extraction due to lower perfusion and slower flow of blood through the capillaries?
I think that could well be accurate.