Answer... It depends.

The US courts have ruled it a religion for First Amendment purposes. But that is a very specific legal definition of atheism in terms of legal rights for individuals.

In terms of "belief systems about God", it's kind of a grey-area. As you say, as soon as theism is defined, atheism is defined. Because as soon as a concept is created so is its opposite - the P vs. !P proposition. So in a sense, if you consider a "religion" to be a "belief system about God" it can be argued that atheism would be a religion, in a purely academic sense.

In a practical sense, the term "religion" embodies an institutionalized belief-system around God, or the concept of one. Methodist is a religion, and so is Christianity. Shiite-ism (forgive my lack of knowing the correct term) is a religion, and so is Islam. Hasidism is a religion, and so is Judaism. etc. In this sense, atheism is specifically not a religion because it is not any of these.

Why the "stupid people" won't give pause to clarify terminology and definitions before posing their arguments is a testimony of how stupid and stubborn they really are.

This is my understanding and/or my perspective anyway... ymmv.