They had the 8 patients from the UK, with individual IgG samples used for some studies, and two pooled sets of IgG from Sweden - Supplementary Table 2 suggests each pool had 14 people. I'm not sure of the details, but it certainly does look as though the sample size was considerably bigger than 8. And, by pooling the IgG, they perhaps accounted for the fact that not every contributor of IgG had autoantibodies producing the effect.
Often you, Jonathan, know things I can't know about what has gone on behind the scenes of some research, sitting in my house a long way away from where the research was done. But, from here, there looks to be enough to suggest that it is worth allocating research funds to look further at the ideas.
The thing that bothers me is along the lines of
@Jaybee00's question above. i.e. Where are the accounts of people with fibromyalgia (and ME/CFS if there was a something similar going on in ME/CFS) being helped, even temporarily, by one of those therapies of removing antibodies from the blood?