Oh, it certainly is, but the issue is not what we are dealing with -- the issue is how the ISIS fighters perceive it.

I'm pretty sure their perceptions are closer to an Albigensian Crusader's attitude toward Catharism -- or even your average Chick tract fan's attitude toward Catholicism -- than some shit-kicking medieval peasant's grudge toward the old man down the lane who once scammed him for a folk healing ritual that invoked a couple of barbarous names for shock value. Treating religious opponents as devil-worshippers is pretty much built into the basic structure of (premodern, and some modern) Christianity and Islam, whether or not there's anything to the accusation (though as I note above, the charge is at least as sticky for Catharism as for the Yazidi). The competing presence of a structured religion that's related closely enough to be uncomfortable but not closely enough to be a heresy per se... that's a little more distinctive.