I see this assertion made by some folks that I have to admit, I consider to be of limited intelligence, whether self limiting or limiting is beyond my ability to discern.
But atheism, as a religion. Now, I'm not an atheist, so this really isn't a defense. I'm more of a deist, which I don't think of as a religion either. Religion is belief system. There is no system to deism, less so to atheism.
Without theism, atheism would cease having a meaning. So how can it be a religion? I can see it from the point of view as rights. You have a right to "be" an atheist. Which means what? Well, not belonging to any church or religion, not believing in a god or creator. That's about it. A very diverse bunch they are.