I think though that they probably realized that shit and filth can cause sickness when it's in your blood, regardless of what the reason was that they thought it did that. A soldier on the battlefield would understand wounds get infected and that had nothing to do with demons but with a piece of metal entering the body. So it wouldn't be a big step to think, "Filthy sword will make them sick." Just as the barbarian hordes might not have realized what the plague was or its root cause, but they did know being around the corpses could get you sick. Because they knew enough to throw them over the city walls to kill the people inside.

There's all kinds of things we do that work despite our not understanding the real reasons. Tech often precedes science in that respect. Now, that Ellis is phrasing it in a modern way, I'll grant that, but that's kind of the whole tone. This isn't really a character so much as a narrator, not different from Larry Gonick except in personalizing it.