If I had to guess, I'd wager that the fire witch tribe was never actually wiped out, and the fire witch tribe (or, at least one fire witch) is a sort of a "woman behind the curtain" to the church Hunters.
@Boomburst The way I took it was what you said is one rumor though it was witches in general not one tribe, and the alternate rumor is the church tried to destroy the human world.
The transition did make it seem like the blame was on one tribe. I had to read it a few times myself when I got to that part.