The third person narrator—who is Hersey—goes above and beyond to leave himself out of the story and present the stories of his six subjects in an objective, journalistic way. There is no "I" voice here, and there's pretty much no editorializing; except for a couple of moments where he sneaks in some judgment—for example, when he ends the book with a passing reference to the world's memory getting spotty—his descriptions and perceptions are totally rooted in his subjects' memories.
An example of one of Hersey's few editorializing moments occurs with Miss Sasaki. Describing what happens when the girl's friends finally find her, Hersey seems to get a little judgy about how these friends treat her (and with good reason):
On the third day, August 8th, some friends who supposed she was dead came to look for her body and found her. They told her that her mother, father, and baby brother, who at the time of the explosion were in the Tamura Pediatric Hospital, where the baby was a patient, had all been given up as certainly dead, since the hospital was totally destroyed. Her friends then left her to think that piece of news over. Later, some men picked her up by the arms and legs and carried her quite a distance to a truck. (3.37)
In the passage above, you might have noticed that Hersey largely sticks to the facts without dressing them up/commenting… but when he observes that Miss Sasaki's friends basically dropped a huge load of bad news on her and then "left her to think that piece of news over," you can't help but get the sense that he thinks it's super crummy that they peaced out right after basically telling her that her entire family was dead. Sure, he doesn't say a lot, but it's kind of a dagger of a comment. But that's seriously about as far as his "commentary" goes.
As a result of Hersey's attempts to remain objective and journalistic, we sometimes don't push too far into the six subjects' inner lives. Instead, there's an emphasis on action and "what happened"—you know, history-type stuff—rather than the more subjective stuff like emotions and motivations that would involve making some non-journalistic leaps of interpretation/reading between the lines.