Real Ns would disagree.

Why is that relevant? Real Ns weren't good rationalists after all. If the existence of Js really made them suffer (which it most probably didn't under any reasonable definition of "suffer") but they realised that killing Js has negative utility, there were still plenty of superior solutions, e.g.: (1) relocating the Js afer the war (they really didn't stand in the way), (2) giving all or most Js a new identity (you don't recognise a J without digging into birth certificates or something; destroying these records and creating strong incentives for the Js to be silent about their origin would work fine), (3) simply stopping the anti-J propaganda which was the leading cause of hatred while being often pursued for reasons unrelated to Js, mostly to foster citizens loyalty to the party by creating an image of an evil enemy.

Of course Ns could have beliefs, and probably a lot of them had beliefs, which somehow excluded these solutions from consideration and therefore justified what they actually did on utilitarian grounds. (Although probably only a minority of Ns were utilitarians). But the original post wasn't pointing out that utilitarianism could fail horribly when combined with false beliefs and biases. It was rather about the repugnant consequences of scope sensitivity and unbounded utility, even when no false beliefs are involved.