Napalm Death are an interesting one! I think there's an interesting argument that when the punk political elements are added the songs become very serious, all the rage and screaming and death and skulls of metal suddenly gains a legitimate reason. Metal is absurd because none of that stuff really has a purpose, it's just there to be metal because that's what metal is, when the politics is thrown in suddenly all of that becomes legitimately dark. Where metal has no motive for the excess, punk on the other hand wears its motive quite openly. Combining the two definitely turns metal in a different direction to its usual roots.

I'm not sure about my thoughts on the usual anti-war metal, I can think of a few songs that fit this off the top of my head and you're right that they're hard to view as absurd in many ways. Soad does a bunch of this content and is both absurd and anti-war though.