>>23761346He was upset that Oppenheimer presumed to take responsibility for the event despite his lack of authority (which rests with the president).
Truman outright says as much in the movie, so it's bizarre that people get confused by the scene. (OP is memeing of course, but this happens irl).
You have to remind yourself that 25% of people have an IQ below 90 and (evidently) can't comprehend basic statements or retain plot information. I guess if you're a moron every film is just a random series of images and sounds. Makes me wonder how smart people perceive average people like myself.