Telling someone with "Impostor Syndrome" to "act like you've made it, because you have" is like telling someone with severe clinical depression they "just need to cheer up."
I'm suffering from Impostor Syndrome pretty bad, after moving across the country alone and going from being a big fish in a small pond, to a small fish in a big pond. Every day I am killing myself over this, and it's not healthy. I know I should accept the fact that I'm doing well, but I haven't yet.
We all have different influence and interests in our lives. You can know more about X than anyone else in the room and not even know that Y existed. The important thing is to make sure you interact with enough Ys to broaden your horizons.