"We are a way for the cosmos to know itself" is saying that humans, unlike any other animals on Earth, have reached a level of mental acuity where we can look up at the night sky and ask questions about it. We do not need to be masters of cosmic knowledge for the statement to be true. As long as we have both the ability to question the Universe and the desire to answer those questions, we will be the Cosmos' way of knowing itself.

The statement doesn't refer to simply being aware of the Universe, but rather our ability and drive to learn as much as possible about it. Without sapient life, the galaxies, nebulae, and planets would go about completely unnoticed, unstudied, and unknown.