Defining "Atheism"

[Welsh cake]

Those who respond to God-claims with an honest "I don't know" are the indeterminate, not the "non-existent".

"I don't know" is an epistemic response, reflecting agnosticism. The issue is atheism—and, as per Adrian's point, using the principle of excluded middle to argue that there is no third alternative (i.e., if theos is X then atheos is ¬X, which exhausts the category of X.) Agnosticism, as a subset of both atheism and theism, is necessarily a different category (i.e., if gnosis is Y then agnosis is ¬Y, which exhausts the category of Y).

No, it's not reflecting agnosticism, that's the very separate issue of what is knowable or unknowable to a degree of certainty. "I don't know" is not a position, it is a response that acknowledges an absence or lack of coherentism. Whenever someone responds over and over again to claims, regardless of whether these claims are making propositions of there being god(s) OR not being god(s), with a "I don't know" it tells you nothing about their beliefs. It can be used by many who require justification to answer the question, including some verificationists, as a regress argument which is a fundamental problem in epistemology because they are not making any claim to knowledge therefore this exchange can carry on endlessly in a debate.