Intellectuals, as defined for this book, are people whose entire trade is in ideas. Not people like engineers or physicists, who have to put the ideas into action and see when they fail.
It's a painful history. The history of rejecting outside evidence -- such as praising the USSR during famine. The imposition of visions that assume that people's mundane and detailed knowledge of their own lives is not relevant next to the ideas of an intellectual who does not know them. The suppression of facts and the hyping of ill-found stories in the service of ideas. And much more.