It is an epistemological basic that everything you know/believe to be true in your subjective knowledge may or may not be true in objective actuality.
Let's say in actuality there is a God. The following positions:
1. God exists. (True in actuality).
2. God does not exist. (False in actuality)
3. I don't know whether or not God exists.
If in actuality, there is no God:
1. God exists. (False in actuality.)
2. God does not exist. (True in actuality.)
3. I don't know whether or not God exists.
In actuality, all are either with God or without God/Godless.
Thus, no matter what claim you make, actuality remains the same.
Correct that the claim that "I don't know" is one who does not believe there is a God. However, there is no reason to want to group the undecided into an ism. They do not fall under atheism as much as they do not fall under theism.