I don't believe it's true that "you are whatever you tell yourself you are". Obviously you don't change if you tell yourself you're this or that because it takes more than that to change yourself. And also, this sounds like something you'd read in The Secret: all it takes to make something happen is to believe in it or think of it.
However, this is most likely not what Daniel really meant. In the same paragraph, he refers to a person who literally doesn't believe that they have the idea-generating ability. So then, what he really meant is related to someone's belief in their possession of ability to do something or make something happen. In other words, their belief in their power.
So, a better articulation would be: your belief in your power affects your power. Which makes sense; if you don't actually believe you have power, you won't try to use it, and you'll have effectively crippled your power.
This comment might seem trivial at this point, but I think the principle behind it is important. Namely, the principle of precise and accurate articulation of thought. The new articulation I presented is accurate, empowering, and useful. A person can directly use it and it will have a positive effect. The old articulation is confusing, inaccurate, and untrue, which I don't intend to say as an insult or an attack, but rather intend to say as an accurate description of how I understand it.
Now, you might think, "I understood what it meant given the context, and so did you, so what is the point of accurately articulating it?" I believe it is true that I understood it given context because I believe that's how I came to form the new articulation. However, the effect of understanding an idea from an inaccurate phrasing or wording is that the person doesn't have a proper articulation to give form to the idea, and so it stays formless, in the subconscious, and cannot be directly used, and the person isn't consciously aware of it. The only effect it has is that it influences conscious thought and requires annoying deconstruction of thought patterns to catch if it's a bad idea ("Why do I think that?").
In contrast, proper articulation increases a person's consciousness, self-awareness, power, and gives life and color to their minds as thoughts are being fully expressed (expression leads to colorfulness, lushness, etc. etc.). So your PSA for today is: strive for precise, accurate articulation in all of your thoughts. :)