I think the most charitable interpretation would be that people expressing that view are deeply self-conscious. They are afraid if they followed the advice in the book, they might be perceived as manipulative and they want to avoid that possibility. They hide from that fear by insisting that it actually must be manipulative.

Outside of that, I can only see less charitable interpretations. e.g. The idea that the only reason someone could ever compliment another person is to manipulate them says either that the person holding the idea literally can’t imagine interacting positively with someone for non-selfish reasons (psychopathy) or that they hold such low opinion of the rest of humanity that they believe no one else could (misanthropy).