"one of the least" != "no military component". You're absolutely correct that "There was no such thing as a non-militarized city in Japan at the time". Hiroshima had been a city that refugees had been fleeing to. It is simply true that for its size it was one of the least militarized cities in Japan at that time.
In something that's rather sickening, and one *hopes* was accidental but suspects that it wasn't, the US had been leafletting Japan in the weeks leading up to the atomic bombings, warning them to evacuate "Otaru, Akita, Hachinohe, Fukushima, Urawa, Takayama, Iwakuni, Tottori, Imabari, Yawata, Miyakonojo, and Saga" There was no mention of Hiroshima or Nagasaki. Anyone who listened to the US leaflets walked into the bomb zone.
Byrnes (claimed source of your quote) was an atomic bomb radical within the government. He wanted to threaten to bomb the Russians to get better success in the postwar negotiations too. But if you search for your quote online you'll find only half a dozen hits. It appears to be an urban legend. Claims that "The US leadership also feared the planned invasion of Japan by the Soviet Union" are somewhat true. The US had been trying for a long time to get the Soviet Union involved, but started having misgivings. Various people were concerned to varying degrees about the potential of Soviet involvement.
You're free to disagree with the US military's own postwar analysis of the Japan situation (the Strategic Bombing Survey). But that would be what most people would call "revisionist history".
The fact that there was a coup attempt after the emperor tried to surrender just drives home how little effect the atomic bombings had. The War Cabinet had steadily been shifting more to the side of the doves but was split down the middle, three-three on whether to accept an unconditional surrender. The emperor had been working in secret to negotiate an unconditional surrender, including making preparations to send his son to offer it, but had been delayed by Potsdam. After the atomic bombing of Hiroshima, there was literally no change in the view of any of the members of the War Cabinet, it remained a three-three split. The hawks considered it just another entry in the list of horrors that Japan was experiencing. The Potsdam declaration had been made just two weeks earlier. There hadn't been an imperial conference since the Potsdam Declaration to discuss it. The Imperial Conference on the 9th-10th. It was at this conference that the emperor made clear that he had wanted to accept the Potsdam terms. But it is clearly documented that he already had by that time supported accepting the Potsdam terms, even before the bombing.