No, it means that you don't know/are not certain. If 100% is certainty than 0% "I don't know". Since baby's don't know, they are Agnostic.
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.
No, I still affirm that to lack belief in something you need to know what that something is..
You've already admitted you lacked belief in magical unicorns made of meteorite teapots living in my dog's butt before you knew they existed, so you've already confirmed yourself wrong. Do you need to know what a merkin is to lack one? Do you need to know what a digeridoo is to lack one? In what sense is it required to know what something is to lack it?