I think as the Merriam Webster link shows atheism can be a religion for some people.
Quote:3: an interest, a belief, or an activity that is very important to a person or groupIn that sense it's no different than someone who might say "Hunting is my only religion." It's an appropriate and common usage of the word. I have no idea how many people put that kind of religious importance to atheism, some certainly seem to, though. For me gods are in the same pile as ghosts, goblins, and fairy tale creatures. No more interesting than Pinoccio, but for the religion attached to them.