Yes.The two groups of 18 are just the "severe" ME patients and 18 matched controls selected for this specific metabolic investigation. Their previous paper showed 23% meeting any one of the three criteria and 8% more than one criteria at six months. Still a lot, but not 50%.
4501 - the prospectively recruited students
238 - the students who got IM during the study period
6 months after infection:
55 - students who met the requirements of at least one ME/CFS diagnostic criteria
>> 20 - students who met the requirements of more than one diagnostic criteria
157 - students who were asymptomatic
26 - students who had symptoms but did not meet any ME/CFS diagnostic criteria
So, two of the 20 students meeting multiple ME/CFS criteria weren't included in the study. This study does not set out the numbers of students - I had to look at the previous study (linked in a post above). I don't think this study explains why only 18 students were involved in this study. Probably the missing 2 just weren't available for logistical reasons.
But yes, I share the concerns of others that this very odd grouping labelled 'severe ME/CFS' might be a post-hoc creation to suit the data. Why would you not just choose (before data analysis) one criteria you believed was reasonable and analyse the data based on that? Or report results for each criteria separately. I'd really like to see the results for all the students reporting PEM at 6 months, and for all the students symptomatic at 6 months.