By the time Steinberg's book came out, this story had long been common knowledge. Here is one example of how this story was mentioned in a routine concert review published by the New York Times within a year after Steinberg's book:
There is no way Steinberg, with his impressive music credentials, did not know about the origins of the composition's title. Which means that he simply could not resist the satisfaction of repeating a conveniently anti-American myth. In addition to being dishonest, the much credentialed Steinberg also had shown himself to be a rather dim-witted writer on music. Even if Penderecki himself had been in Hiroshima at the time of its bombing, and had miraculously survived the explosion, it still would be sheer stupidity to speak of Penderecki's tone clusters as being about (let alone as "documenting") the event. If music could have extra-musical semantic content, there would be no need for obnoxious titles, nauseating dedications, smarmy program notes, and countless pseudo-musicological factoids in books, journal articles, and newspaper reviews - all anxious to remind us again and again that a given piece of music is really "about" such-and-such extra-musical thing, event, or concept.
Finally, let us not forget about the incompetent (or dishonest) assholes at the august Oxford University Press who sent Steinberg's book to the printing plant without having challenged his pseudo-musicological fabrications. And while we are at it, lets also note the ignorant (or dishonest) pianists Andre Watts and Richard Goode whose laudatory blurbs adore the book's dust jacket.
 Steinberg is not worth digging deeper, but I suspect that, along with many American leftist "intellectuals", his heart never bled for those hundreds of thousands of American boys killed by the Japanese in the Pacific. After all, when you pluck an 18-year old boy from a farm in Nebraska, give him all of 30 days of basic training, put an M1 rifle in his trembling hands, and ship him to the front line - well, the boy ain't a civilian no more, right? He is now a professional warrior whose death (even when multiplied by hundreds of thousands) should never budge the arrow of a firmly anti-American moral compass...