I'll note that I have a physics degree. My undergrad advisor and several people I've worked for had worked directly under those scientists, though post project. Including my time at the two labs not in New Mexico. I'm saying this to help convince you that I know what I'm talking about here:

That story is wildly overblown. No one really thought this would happen. But given the consequences of being wrong no one wanted to trust themselves. So what they did is keep asking each other to try to do the calculations independently and see if they would get the same results. The idea came from Teller who was researching stars, which do have a runaway reaction. In the movie Oppenheimer says "nonzero chance" and that's accurate. He really did mean "we aren't confident that this would get impossible" but they were very confident the likelihood was almost zero.

The reason this it's important is because it flips the story on its head. Had they thought there was a meaningful chance and moved forward then the story represents how brash they were and careless. But instead it is a story of where they were considering and taking seriously something they thought was a near impossibility and yet they took great effort to ensure that it was actually impossible. That's now a story of how careful they were. It's the exact opposite of being brash.

Mind you, this doesn't contradict what I said before. They did know the bomb would be powerful, but I doubt many knew how powerful until near or at Trinity. And I'll tell you, unless you can do the calculations then you really don't understand what those things actually do. It is so much more than an explosion. It is more than the radiation. There's a reason so many could no longer be blind to the consequences. To this day the skeletons are out of the physicists' closet because we all know we are capable of unknowingly following the same path. When you learn what that bomb does you hope you're at the center of it

Would you say they were significantly more concerned about nuking the entire atmosphere than scientists were "concerned" about creating a small black hole that would swallow earth by turning on the LHC (AKA not at all)?

The stories have the same vibe to me.

Wow what an amazing follow up! I’m thrilled to hear this anecdotally with your personal commentary. I always go down the rabbit hole and take everything at face value, so I appreciate the color you added to the story.

Any good books or documentaries you recommend?