They did not get vaporized in the fireball. We have survivors at the hypocenter. Intense burns and radiation poisoning killed most plus conventional blast damage mechanisms. Half the acute deaths happened after the first day.
Like the beatles joke, "were the atomic bombs the worst atrocity of all time? They weren't even the worst of that war!"
I mean, we don't really need to rank awful things, we can acknowledge anything beyond a certain level is awful without comparing them, but lots of awful things happen without involving nukes.
Fortunately the novelty wore off fair swiftly.
The application sure, but the novelty of it being nuclear rather than conventional kept parties interested in it for quite a while!
that's debatable. like, worst single instance atrocity? possibly is