I don’t know how you could read that book in a modern frame and get syrupy insincerity out of it in good faith, but it is true that many people read this type of book looking for shortcuts and hacks, so they are not looking for the good faith reading. If you read the rest of NH’s work, you will find that his philosophy is firmly based on genuine good faith and delivering disproportionally high value to the people you interact with. (As well as some very 19th century metaphysics you can safely ignore)
But yes, many people read that book, ape the ideas, and act like asshats. It’s hardly a failing of the book.
Manipulation seeks to get someone to do what you want against their interests.
Persuasion seeks to align their interests with your interests in a mutually beneficial manner. The act of speaking to their interests by definition takes their well being into consideration.
“You’re such a generous person. I know you love being there for friends. Could you help me move Saturday?”
It's an example that of course sounds ridiculous to anybody that reads it, but it's essentially what the book teaches. Gather information not even about the person but how the person feels about themselves and then use that as a tool to make them feel good when you're talking to them.
I suppose it's also not helped that many of the people I've met who use this and admire this book have psychopathic traits. They're very good at faking caring but actually can't. It's a cold calculation to them and admittedly it's very effective at getting them ahead.