It was posted in the Religion group because religion is not incidental to the story. Religion (not atheism) was a central motivating factor for several of people involved, though certainly not all. Similarly, it was not posted in the Atheist group because we had zero reports of people motivated specifically by atheism who participated or wanted to claim the country in the name of atheism. Of course some of the insurrectionists very likely were atheists like the people you mentioned. Few groups of any size are asshole-free.
The article doesn't by any stretch label all churches as terrorist organizations. I would say it had a couple of points: First, we have an obviously active Christian Nationalist/Dominionist movement in this country, and articles calling this to broader attention can help us keep an eye on these groups. Second was to point out Christian privilege by contrasting how differently this story would have been portrayed had groups of Muslims prayed to Allah during an insurrection.
I am not interested in bashing religion. I AM interested in bashing those who use religion as an excuse to oppose democracy, and this article illustrates that those people are among us.