I'm convinced by mathematical arguments that utility should be additive. If the value of N things in the real world is not N times the value of 1 thing, then I handle that in how I assign utility to world states.

I don't disagree. My choice of slogan wording - "utility is not additive" - doesn't capture what I mean. I meant only to deny that the value of something happening N times is (N x U) where U is the value of it happening once.