How is the term “agnostic atheist” an oxymoron?<quoted text>
Agnostic atheist is an oxymoron.
A true atheist does not believe in the existence of deities. They just don't.
So how could someone that doesn't believe in the existence of deities also not "know for sure" if deities exist?
Nothing in the rest of your post supports its first sentence.
The second sentence is precisely correct. An atheist does not believe in the existence of deities... and that's it. That's all that's required to be an atheist >>> A lack of belief that deities exist; Rejection of the theists' assertions that deities exist.
Agnostic means “without knowledge”(pertaining to the existence of deities).
Atheist means “without belief”(ditto).
As much as a person may shy away the label, if they don't believe at least one deity exists, then they ARE an atheist... no ifs, and, or buts about it. What they don't know has nothing to do with it and goes without saying... everyone's agnostic.
Your question isn't logically stated. Belief and knowledge are two different things. The latter can be considered a subset of the former. If someone knows something, then they must also have a belief about it. Conversely... they don't have to know something to have or not have a belief about it.
You don't know there's a god... but, you believe one exists.
I don't know there isn't a god... but, I don't believe any exist.