I don't think this is a tiny sample size. I think genetics studies generally need enormous samples when they are testing massive numbers of genes or the whole genome, to have enough power to detect effects after multiple test correction, and they have to correct for thousands or millions of tests.
Here it's just one test, so I think this is actually a fairly large sample at 458 people.
Seems sensible to me within this study, but isn't the whole idea based on a study with far less participants were they looked at how a subset of genes differed in a GWI(+) and GWI(-) cohort?