Perfectly put. I am concerned about this claim: it would explain how they get such a small P value from the tiny sample, but that P value does not tell you how likely the finding is to hold up in the wider patient population. On the letter is what we need to know.
There is another issue. IIRC, statistical theory says that you need a sample of at least 20 patients per group to have confidence that the sample is representative of the wider population (which is why you see very few studies published with you with fewer than 20 individuals per group).
I was mulling this over and I think *theoretically* you could get the p values mentioned with an N=18 sample. I agree with the above that sampling hundreds of cells gives you more confidence of the deformability value for a given patient/control but does not provide greater confidence of the differences in average deformability *between* patients and controls. However, if for example all nine patients had deformability lower than all the controls with no overlap between the groups, the chance of getting that result if the null hypothesis was true would imply p = 7.6x10^-6 (based on the heavily oversimplified approach of treating each patient/control sampled as being a 50/50 coin toss between being in the 'high deformability' and 'low deformability' groups). So I could see how one could get a very low p-value if the results were massively clear-cut with no overlap. As you say, we need to see the results.
The wider question is your point around whether the sample is representative of the patient population: the severely ill study as benefits and drawbacks. The benefit is any 'underlying signal' might well be stronger in the severely ill than in mild/moderate ME/CFS patients; the drawback is that severely ill patients are likely to be much more deconditioned and on other medications meaning that any findings may be merely a result of that instead.
ETA: and the real biggie, for all ME/CFS research - we don't even know if severely ill patients have the same disease aetiology as other patients!