In fact, atheism isn't anti-religion, in dictionary sense it's just the lack of belief in gods. So, if they don't do that, they are atheists.
Antitheism is antireligion. But your idea sounds realistic, that there logically isn't an opposition to religion when it doesn't exist.
It's similar among Zarvans... Though I'm writing their history, I still consider them to have formed naturally by evolution, and their system being formed in a part of the history of the universe. I only made Eastautarkia religious, or maybe I didn't, but only considered it.