If your argument rests on dictionary definitions, and common language use, you have no real argument. Because we humans use language as much to confuse and obscure and deny reality as we do to try and clarify it.
OK, so you admit that you are using your own definition, abandoning the actual definition of the term and how it is commonly used. That's a good start.
So, here is one reason why I think it is more appropriate to include those who merely lack (or are without) belief in the existence of God:
There really is no reason to believe that Atheism MUST be a position or a held belief. The word itself means "without theism". The prefix "a" means "without" or "to lack" and "theism" means "the belief in the existence of God or gods." So, why would atheism need to be a position rather than merely the absence of a position.
Also, the vast majority of self avowed atheists are not outspoken and they don't claim that God is an impossibility. Obviously, that can be hard to see unless you go out of your way to talk to atheists outside of debate forums, as those in debate forums will almost certainly be outspoken. They do not believe that God does not exist. They just haven't been convinced by the evidence that God exists. Many, in fact, don't see any verifiable, trustworthy evidence that isn't purely based on personal experience, logical fallacies (like god of the gaps), or anecdotes.
Finally, "theism" is an extremely general term. It includes deism, monotheism, polytheism, pantheism, autotheism, and value-judgment theisms. It also includes the belief in an almost infinite number of different deities throughout human history. So, logic would dictate that "atheism" be even more general. It should include weak atheism, strong atheism, agnostic atheism, gnostic atheism, etc. The only way to accomplish this is to include those who merely "lack belief in the existence of deities" in the definition of "atheism".
So, now it's your turn. Why do you think atheism must be a position (iow, why do you think that atheism should only include strong atheists)? Why do you think it should be so specific when "theism" is so general, including belief in countless deities throughout human history?