a) Nature Neuroscience is one of the best journals in the field. I guarantee that the methods of this study were put through the ringer before it was published.

b) You don't typically measure skull characteristics when you are doing ERP, which was the method used in the paper. The 10/20 system of electrode placement means the relative distances matter, not absolute measures.

c) I totally agree that the sample size was off to make a statement with this kid of gravity. While it is large enough to get significance you do want a more random sample from the full population - perhaps form several urban and rural areas - to be more conclusive.