I haven't experienced this whole exchange in-game because I have never actually gotten to the underdark. The one time I actually managed to get through the underground thing beneath the goblin fortress, I couldn't figure out how to get through there and into the underdark proper, and frankly hearing about things like the Bullette fight have made me really not want to bother. So I don't know the entire context of the situation. If Duergar are vulnerable to poison and in D&D rules they shouldn't be, then yeah, that's not good at all and it is emblematic of a larger issue. But if it's simply a case of a character not knowing that his plot wouldn't work, then I think that's actually kinda fun.
