There are two choices here 1)Christian (Christian in Spirit) 2)Troll. There is no other choice.
I believe he's not a Christian. I think it would be a sin for a Christian to deny their Christianity. I know a nonbeliever who defends Christianity out of respect for his Christian mother. Maybe it's like that for doombug. Also, he seems to have a strong dislike for atheists. So maybe it's the enemy of my enemy is my friend kind of thing.