It points out the fact that there are different aspects of and kinds of "smartness", and the fact that individually they DON'T necessarily preclude dumb actions or ideas or the commission of "dumb things". It specifically mentions some well-known idiotic actions and beliefs of people who are generally considered smart but aren't "above" dwelling on their own individual concern regardless of their knowledge or background (to me the most obvious example it is the Nobel prize winner Linus Pauling who decided to push vitamin C as a cure-all – including being a cure for cancer, despite the fact that that was not even vaguely related to his celebrated research. (I believe he ended up dying of cancer…)
Anyway, there is a difference between being "smart" and being able to demonstrate proficiency in absolutely everything that you address. Only one of those is conceivable, but I'm getting off point. The point is, WHY smart people do dumb things, and the book suggests several reasons. Primarily, it's important to refine our meaning of smart.
I found the last two chapters that old and long, and I need to skim the end, but overall good things and techniques were examined.