[libraryofbabel]

People that know more about nuclear physics than I do already answered, but I’ll just say that:

1) It’s easy to think about the past in terms of what we now know, and it involves a real effort to put yourself in the shoes of the people living at the time and to imagine the “fog of war” in what they knew. In 1945 nobody had ever tested a nuclear explosion before and there was still all sorts of uncertainty about it. And as one of the other commenters pointed out, in particular there was a lot of uncertainty about how fusion worked.

2) The center of the Trinity fireball did in fact produce hotter temperatures than had ever existed on Earth before. Temperature and energy being different things.

In some sense the final experimental proof that a nuclear explosion would not set off some unanticipated new chain reaction that would destroy the earth - unlikely, but hard to completely disprove - was Trinity itself. Only after Trinity is it obvious and completely proven how the physics actually worked and obvious that there were no additional reaction pathways that got missed. That is a disturbing thought.