They're not talking about extrapolating from one mistake though. They're talking about drawing one observation from a population of an unknown distribution. At that point that single observation the only estimate of the quality of the entire book. If you decide to not sample further (that is find other chapters in the book of which you are an expert) then the conclusion that the book is trash is the only rational one.

(If you want to read more about this then google "single observation unbiased estimator")