Um, that's simply not true. Atheists don't believe in the supernatural full stop. That criticism includes both god and satan by definition. It feels like god is attacked more because you get more upset by gods existence being challenged than satans. That leads atheist writers to say god more. The criticism lands on both because the proof applies to both. You are arguing the structure of thinking that leads to people writing he/she to be pointedly inclusive. We both know what you meant by he in English and if that still rubs you the wrong way they is the correct word to use. For a more literal and direct disproof of your point, Carl Sagan called his book "The Demon Haunted World."