But not MacArthur?!
MacArthur did not need to know, as he would have no operational authority over the activities of 509th Composite Bomb Group. Apparently all information about the Manhattan Project was disseminated by one-on-one oral briefing, and only to those people who needed to know. Captain McVay of the USS Indianapolis did not know what she was carrying to Tinian. Only Col. Tibbets knew what the Enola Gay had in her bomb bay when they took off on August 6, none of the crew knew.
In military circles outside of Gen. Groves staff, it was a very short list of military officers who knew. And why should they have known? Nobody knew the bombs would actually work; not even the scientists who designed them. (They were pretty sure the Uranium bomb would “work,” but they had no idea what the yield would be. The Plutonium bombs were a guess until Trinity.) The military had to plan to finish the war as though there were no atomic weapons.