Why should glass bullets and other odd items (usually not used as ammunition) harm a fext?
A fext is a revenant that is impervious to most damage, superman style, but glass it its kryptonite, why?
Perhaps non-translucent materials suitable for use as projectiles reflect the person's intention back on him who wishes to harm the fext, and glass, which is translucent, does not? One would have a neat pair of binary opposites here: metaphorically reflective and non-reflective, and physically non-translucent and translucent.
This is just a guess. It could also be something to do with the amount of knowledge that has gone into making the glass, especially if it is a special kind. As an amorphous or non-crystalline solid, glass is weird - it is practically its own state of matter - and this too might be relevant, analogously to the "neither day nor night" nature of twilight.