Well you don't have to literally believe everything you hear, for example Aristotle had a great theory of virtues, although he had a weird view of females intelligence.

He basically said that men can do things and understand why, and women can do things but it's impossible for them to understand why.

Should we bash his theory of virtues because of that? No! In fact some of his virtue theories have been modified to fit modern views and have even been used by lots of feminists.

The same goes for this idea that our thinking is a delayed manifestation. I simply believe in it because of my own experience, if you don't have any (repeated) experience that gives you a reason to believe, then it isn't rational to believe in it.

However I like to see another reason to why positive thinking works. It kind of works in the same way as a smile on a party. If you walk in to a party while you are smiling, people will look at you and behave around differently compared to if you walked in depressed or nervous. Also as you smile you are releasing endorphins, natural pain killers and serotonin which makes you feel good.

With a positive attitude you will start a chain that will make the world around you both give you positive feedback and give you a better chance for your desired goals than a non-positive attitude.

So ignore the spiritual idea of positive thinking and keep the important message.

Because what you are is indeed, atleast for the most part, a result of what you have thought.