Like any category, they get added as creators get added for whom they are relevant. Thus, “Muslim creators” came about when I added G. Willow Wilson, “Jewish creators” came about almost as soon as I started going through Golden Age creators, and so on. So of course any religion is welcome to be added as soon as there are articles for creators who profess them.
And for simplicity’s sake, I would put “Atheist creators” in the “Creators by Religion” category, even though it’s not technically a religion.
Also, I’m sure that several of the creators who do have articles already are atheists or other religions not listed, I simply make no presumptions unless I come across an explicit mention of it somewhere else, but religion is frankly the lowest priority of information I try to find out about a creator. If a creator’s religion is listed, it’s either because I came across it in an interview, on their Facebook page, or Wikipedia article. It’s not the kind of information I contact people about. To me, that would be like asking someone’s sexual orientation/gender identity.