[Well, considering that they are just the Japanses pendant to demons, monsters and fairies,.... That's not all that off to assume. Especially since eating others has long been a staple even among the different species of them. Douman's sense for what can and can't be said has just been worn down by manga comedy violence living so long among them and getting used to how much they can actually take without dying.

....and maaaaaybe he's also pulling Miller's leg a little by now.]

I'm thinking of unorthodox solutions. The best way to break a system is to confront it with scenarios it hasn't been prepared to deal with. Kind of like how there can't be a war if just everybody refuses to participate.