That's solipsism, not existentialism.

Existentialism is much more to do with the idea that you live in the moment, and your actions and decisions are not restricted by your actions and decisions at any previous time. "Today is the first day of the rest of your life" is really a kind of existentialism.

That's what I meant by saying that declaring oneself to be an existentialist is not really the kind of thing a real existentialist should do.