It just doesn't make sense. So it is immoral for engineers to design weapons? That's like saying it's immoral to butcher an animal. Sure go ahead, ban butchering animals in your society so everyone can feel good about themselves while they import their meat and thereby outsource the butchering instead. Or you say that everyone just stops eating meat? Fine that works but here the analogy breaks, because everyone won't just stop needing weapons.

What I can agree with is that placing a moral cost on engineering designing weapons in some way increases the actual labor cost of weapon R&D, I guess.