For 104 of the 185 nations, no studies were available. In those cases, the authors have used an estimated value by taking averages of the IQs of neighboring or comparable nations.
I'm glad to hear no one actually spent their time testing the intelligence of the Sudanese. It's an interesting concept, but this book compares IQ tests that had different methodologies and scales, and were conducted at different points in time. Also, some of the tests were done on small population pockets, not a national average. This would render the results unreliable if the authors' intent was to do country-to-country comparisons.
Some peers attacked it, others praised it. You can see this on the Wiki page. Their overall thesis wasn't of all that much interest for me when I first saw it. For the record, I have never read this book. I saw it while reading up on evolutionary psychology.