No, it doesn't. While the discussion of the definition of religion would result in its own holy war, suffice to say that defining religion based of the involvement of the supernatural is cyclical logic used to discredit any opposing religion. "Pagans believe in the supernatural, the sun isn't a mythical being! Only belief in my God is grounded in reality" is not functionally different from "Theists believe in the supernatural, there is no mythical God! Only my belief in the lack of anything beyond my current understanding is grounded in reality."

If we can't agree on a definition for religion, then saying I belong to a religion is a fairly meaningless statement no?

Linguistics are the crux of all philosophical discussions

Atheists don't believe they know everything, they just believe that people are full of shit when they talk about Gods or similar. It doesn't require any belief in the existence of or lack of supernatural things, just figuring out that it is much more likely that people are manipulative liars than that they got some profound message from some higher being.