This is a flawed analogy. The conceptual basis of nuclear weapons was well understood as soon as it was learned that the atom has a compact nucleus. The energy needed to bind that nucleus together gives a rough idea of the power of a fission weapon. If that energy could be liberated all at once, it would make an explosive orders of magnitude more powerful than anything known.

It was hard to predict when or if such a thing could be made, but everyone knew what was under discussion.

Compare this to AGI, some vaguely emergent property of a complex computer system that no one can define to anyone else's satisfaction. Attempts to be more precise what AGI is, how it would first manifest itself, and why on earth we should be afraid of it, rapidly devolve into nerd ghost stories.