It exactly means that. You can't know whether or not you know something unless you've thought about it. Babies can't do that and they lack a belief in god(s), therefore they are atheists. Try again.
Wrong.
Did you know there is a blue star called: "Alcyone"? (if you do, insert something else you don't know, how many teeth a Tyrannosaurus have etc.)
Now, how can you not know this, if you have not thought about it? (have to be something you have not thought about until now as well, of course)
Hence, your logic is flawed. Try again.