The trinity tests weren’t even close to Japan’s shore… yes, spies would’ve seen it, or heard about it, but regular army people, generals, etc. and the emperor would only know a second or third hand story.
Compare that to walking down the street and seeing a giant mushroom cloud at a safe but not so far distance, potentially with a large part of Japan’s navy gone in a blink (and maybe a bit of a tsunami as well). Let’s say this was timed such that the emperor himself would likely observe it. We can’t know for sure, and I concede that Japanese culture was very much “victory or death” at that time, but seeing it in person might, just might’ve changed some people’s mind, with a much smaller civilian death toll.