You must be looking at different dictionaries than me for "atheism."
Webster:
a: a disbelief in the existence of a deity (me)
b : the doctrine that there is no deity (you)
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OED: "disbelieves/lacks belief." This is *exactly* what I'm saying. I don't see how you can say that supports your view.
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dictionary.com puts your definition first, but you neglected to mention that mine follows immediately:
1. the doctrine or belief that there is no God.
2. disbelief in the existence of a supreme being or beings.
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Cambridge: I got nothing.
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In other words, in all of these cases, barring Cambridge, the MOST BROAD definition (and usually the top definition) is my definition, as well as the definition atheists generally apply to themselves. Even those atheists who assert that there is no God acknowledge that those who do not are still atheists.
You can also check the etymology of the prefix "a-" for further convincing. (I'll save you some time, it means "not.")
For example, if I am not moral, my behavior is a-moral, does that also make it im-moral? No, it does not. You could think of a million different similar examples.
Likewise, If I am not theist (a-theist), that doesn't mean I MUST assert that there is no God.