In fact, there's no evidence it applies outside the sampled population (students in Prague). What the researchers actually seem to have "discovered" is the existence of population structure in the Czech Republic that manifests in different frequencies of morphological types among lightly- and heavily-pigmented inhabitants.
In contrast with blue-eyed males, brown-eyed males have statistically broader and rather massive chins, broader (laterally prolonged) mouths, larger noses, and eyes that are closer together with larger eyebrows. In contrast, blue-eyed males show smaller and sharper chins, mouths that are laterally narrower, noses smaller, and a greater span between the eyes.According to Coon, "Alpine" and "Dinaric" types differ in frequency between regions of what of what is now the Czech Republic:
While Alpines and Norics are commonest in Bohemia, there is a strong concentration of Dinarics in Moravia, especially among the miners, who seem to form a special group with both racial and occupational peculiarities.To their credit, the authors advance "genetic linkage" as one potential explanation for their results (noting that this hypothesis could be tested by repeating the experiment in different populations), though this seems to be their least favorite of the three possible explanations they lay out. Unfortunately for them, it's also the most likely to be correct.