In the context of the London fire, it's definitely not a small area. A similar fire in, say, Sydney wouldn't magically make a 6 mile radius "tiny".

This. It seems pretty irresponsible for Facebook to be running this out in the open with an algorithm which is clearly not doing very well. It should have human approval for each "safety check" event and use that to tune the algos until they're VERY confident that it's doing the right thing. I'm guessing that the radius is so large in order to catch people who might have gone / been in the area. I do wonder if they even have enough tracking / loc data to ensure that they'd pick up a useful % of affected individuals. It seems like they went for "cast a wide net", but in this case it has quickly-diminishing returns.