It's actually pretty clever. We're taking the assertion "Every strong AI instantly kills everyone" as a premise, meaning that on any planet where Strong AI has ever been created or ever will be created, that AI always ends up killing everyone.
Anthropic reasoning is a way of answering questions about why our little piece of the universe is perfectly suited for human life. For example, "Why is it that we find ourselves on a planet in the habitable zone of a star with a good atmosphere that blocks most radiation, that gravity is not too low and not too high, and that our planet is the right temperature for liquid water to exist?"
The answer is known as the Anthropic Principle: "We find ourselves here BECAUSE it is specifically tuned in a way that allows for life to exist." Basically even though it's unlikely for all of these factors to come together, these are the only places that life exists. So any lifeform who looks around at its surroundings would find an environment that has all of the right factors aligned to allow it to exist. It seems obvious when you spell it out, but it does have some explanatory power for why we find ourselves where we do.
The suggestion by D_Malik is that "lack of strong AI" is a necessary condition for life to exist (since it kills everyone right away if you make it). So the very fact that there is life on a planet to write a story about implies that either Strong AI hasn't been built yet or that it's creation failed for some reason.
It seems like a weak premise in that human intelligence is just Strong NI (Strong Natural Intelligence). What would it be about being Strong AI that it would kill everything when Strong NI does not? A stronger premise would be more fundamental, be a premise about something more basic about AI vs NI that would explain how it came to be that Strong AI killed everything when Strong NI obviously does not.
But OK, its a premise for a story.
If Strong AI turns out to not be possible, what are our best expectations today as to why?
I'm thinking of trying myself at writing a sci-fi story, do you think exploring this idea has positive utility? I'm not sure myself: it looks like the idea that intelligence explosion is a possibility could use more public exposure, as it is.
I wanted to include a popular meme image macro here, but decided against it. I can't help it: every time I think "what if", I think of this guy.