I'd say it's because all current atheist writers are human, and therefore suffer from the same basic limitations of brain design as theists. A rational AI could, in principle, form and communicate beliefs without being as vulnerable to the trap. But you probably meant that atheism and theism are both non-fact-based, which is a bigger inferential gap than I'm willing to try to cross right now. (Edit: And I don't get how it's implied by the existence of a middle ground at all.)

In the current freethinker discourse, agnosticism and atheism are considered to be independent properties - "gnostic" meaning "believing knowledge is possible" and "agnostic" meaning "believing knowledge is impossible". There was a very active agnostic theist poster on the forum then known as the Internet Infidels Discussion Board when I was a regular, for example. Even if this were not true, changing the definition of the word "atheism" merely means that the word no longer describes the people you are disagreeing with - not that their opinions must correspond to the new word. (If you are a YouTube fan, QualiaSoup's "Lack of belief in gods" is a pretty good overview.)