What does it mean that the book is not scientific?
There's no scientific justification to its methodology. So we don't know that repeating particular phrases over and over can change one's mind or behavior in a meaningful way.
I bought the book, but I don't think that it being a placebo is the only danger. What if it has negative effects on your way of thinking? What if it has no effect for most people? What if it has a negative effect (e.g. on people with low self-esteem as another comment points out)? What if there's a better way of achieving the same result? What if we don't even know what the right 'what if' questions to ask are?
The low self-esteem is a very valid point. I've had some very strong negative reactions to people telling me to "get over it" in terms of a couple of personal insecurities and faults, and going over the thoughts makes it worse, at least in terms of dealing with the emotions as valid. In fact, thinking about these faults conventionally leads to a deepening of the unwanted behavior.
Having started to meditate in the Vipassana style has started to teach me to accept, but not embrace these emotions. In a less hippy-dippy way, there could be actualization training that helps people get on the right path first, defeating these types of emotions.
I posit that it's something that will make you think it worked (and you'll recommend the book to all your friends), but won't make any actual positive impact on your discipline. Basically if it's a placebo, it would give you self-delusion instead of results.
Actually, the placebo effect is not a delusion, that's why it's called that and not "the placebo delusion." A better analogy would be something that worked despite not believing that it will. Results despite opinion, as it were.
The upshot is that discipline is self-evident. Anything that has an effect on discipline is indistinguishable from placebo. Either you're getting more done or you aren't, there's not a lot of room for "getting stuff done, but in a fake way."
Unless you take time to measure how much you get done (how would you even measure such a thing consistently?) - you might think you're getting more done even though you're not. Kind of like multitasking can feel really productive, even though productivity actually decreases.
There's no scientific justification to its methodology. So we don't know that repeating particular phrases over and over can change one's mind or behavior in a meaningful way.