This doesn’t look like gain of function but overall resistance. Surviving doesn’t make you more of a carrier but, yes, you still might be walking around if undiagnosed.

I think the countries that are able to weaponize a virus are few, and it would be very hard without being obvious. Infecting a population isn’t that hard, so no point in developing immune super carriers. Better to use propaganda to tell people to ignore their experts. Then you have the big problem; you can’t be sure how effective an attack is and if it is, that it won’t come back and bite you. And of course, the rest of the world would be very pissed.

Viruses are not good weapons and they would do more to make your enemy have the resolve to go to war than anything else.