Centuries have passed since the wars of religion ceased in Europe, and since men stopped dying in large numbers because of arcane theological disputes. Hence, perhaps, the incredulity and denial with which Westerners have greeted news of the theology and practices of the Islamic State.
Dude. Dude. Sixty years ago, when your parents were possibly already born, six million Jews were killed in Western Europe. Graeme Wood was already in high school when the Bosnian War pitted Christian versus Muslim, and 20K-50K Muslim women were victims of genocidal rape.
Unless you are going to claim that "ethnic cleansing" or "ethnic conflict" can't be a religious war, in which case, given how whiteness and ethnicity are defined, "religious war" in Europe always needs to be between Christians (or internicine at the very least, but given Europe, this implies between Christians).
(One could argue that since the Nazis were only sort of Christian, WWII doesn't count. I'd argue that wanting to kill people of a specific religion is by definition a religious war, and besides, it's irrelevant, given that most of us should have pretty clear memories of the mass graves they found in Bosnia.)
Basically, at best he's being cavalier and poorly edited. At worst both he and his editor assume that Christians can only understand a group such as ISIS by thinking about Christians murdering each other, because Christians murdering Muslims or Pagan-Occultist-Christians murdering Jews is totally understandable. For what it's worth, I assume the problem is the former. But that also makes the article less believable as a whole.