whilst dictionaries certainly stress the theism aspect of religion, the meaning of the word in dictionaries certainly goes beyond that to include any belief system.
there is resistence from many strong atheists to this ,I think because, they like to insist that their position is a totally objective alternative to a faith position. IMO ,the reality is that atheism is a faith position.
either way, I am not in favour of censuring their position, rather engage in discussion in constructive, respectful attitude.
sadly some take disagreement to be disrespectful.