Our definitions of religion differ. Every human is religious, in other words, has a set of beliefs that frame their perspective on their life and life in general. A religion does not have to believe in deities, it has to be 1.) a cultural construct of many who share the same perpective, and 2.) the capacity to transmit those beliefs to generations that follow. The practice of meditating on those ultimate questions is spirituality, which atheists do as well. Especially if they are thoughtful and not just knee-jerk "there is no god" people. Just like "there is a god, specifically mine" type of folk.

So yes, atheism is a religion in this sense.