Well, a closed source model able to hack half the planet will be a far bigger concern if there aren't enough cheap open source models to harden the existing systems and write software for defense in depth.
> some dude ask "hey plz hack the military"
Military gear, capable of moderate levels of destruction and above, should be air-gaped and also hardened using good open source AI - as a precaution. If that's not the case, state sponsored human adversaries would be able to cause maximum damage even without AI.
Why do you think AI is mandatory here? And why do you think adversarial state actors will be deterred by your banning of your own open AI? They can even promise not to use it, may sign treaties about not using it but enforcing that would still be impossible.
As your parent said, control the tangible agents of destruction, controlling knowledge is the wet dream of many but it's a lost cause.