Not trying to be flippant, I'm genuinely curious. If everyone was as honorable as you and decided to stop working for the military industrial complex - do you think China and Russia would just sit back and say "That's cool - we didn't want Americas land/resources/overseas territories anyway" ?
This is like a thief saying that they steal something because otherwise another thief would steal it. What the parent is suggesting is that engineers around the world should agree to not make such systems similar to how doctors have the hippocratic oath. It may seem naive and can probably never prevent such systems from being built but I think it's worth a try. We have to collectively agree on systems we should not build.
If you are going to use a thief analogy at least make it accurate. Having a moral objection to defense is like objecting to doors and locks (defense against thieves) and so not only are you going to not work on anything lock related you are going to openly advocate no one else work on them or manufacture them as well, leaving us all at risk.
