Earlier quoted context omitted.
At the moment "strong AI" is science fiction so what could you possibly say about it? How do you even know what the "only defense" is?
A super-virus that has a 100% mortality rate, is incredibly infectious, and stays dormant for 1 year before killing the host. There's a fiction (I literally just made it up). Is it impossible to say anything about it? A bomb that is 1,000 times for powerful than a nuclear weapon. Again, currently science fiction. Can't say anything about that either? The fact that it doesn't exist right now doesn't mean we can't say…
Yes. It's not the fact that it doesn't exist, it's the fact that not only is it not clear that it's even physically possible, anything you come up with can just be handwaved away by this mystical property known as "intelligence". Your two examples don't have this problem so they're not similar.
Most of the literature I've seen about AGI is about theories on how to build it, or theories on how to build it safely (e.g. so it has stable preferences that align with human values). Not stuff on "how to defend yourself against a rogue AGI", because there really is nothing you can say about that.