 What is the main message the author is sending by mentioning the tale of Plato’s cave?
There is an ethos of letting the chips fall where they may about the sending out of galleys: Now the book will begin to receive uncontrolled reaction. The effect was, first, to increase the allure of the book (There must be something really hot in there!), and second, to ensure that no one inclined to be skeptical would be able to weigh in at the moment of publication. Herrnstein and Murray didn't do this, so it wasn't until a full year or more after The Bell Curve was published that the leading experts on its subject had a chance to go through the underlying data with care. The debate on publication day was conducted in the mass media by people with no independent ability to assess the book.
 Caution that people who think they see things clearly may just be blinded by what they want to be true.
 Caution that people who think they see things clearly may just be blinded by what they want to be true.  Reminder to be careful what you read.  Caution against the shadows of political correctness.  Reminder that Plato believed in education.