That's very true, especially because the book is a type of sales bible for convincing people to buy things they shouldn't otherwise buy! But if you focus on the broad strokes and try to remain true to yourself, then you'll hopefully avoid insincerity altogether. Wouldn't it be great if somebody wrote a modern-day take on "How to win friends and influence people"? - less sales-y and more relevant to social issues that young people struggle with today?

Plenty of people who haven't read that book come across as insincere, fake, and borderline manipulative.

All thru-out the book, Dale Carnegie tells you that you must be genuine in order for it to be ethical.