You assume we have questions this book will answer and that may or may not be the case, but that doesn't stop me from wanting to see and peruse the book. From the few pages posted above, it was rather obviously written by a person with a bias against German WWII pilots with big scores. A bias which is that openly displayed does not bode well for objective writing / conclusions. Having said that, I have not yet read the book that sentence may or may not be very relevant with regard to Verified Victories. The only way to tell for sure is read it and see if it generates a desire to look at some of the references cited in the book.
Having read the book I don't see the authors (father and son) as biased , and it's a number of eksperten's scores that are analysed not just the one with big scores. I agree that the criteria for when a victory is verified is very strict, but I think perhaps that is to better compare the reliability of the pilots in claiming kills. There is a very big difference between the most and the least reliable., and Hartmann wasn't the worst in this period over Hungary.