I ran across one other potential reference - in August 1945, Campbell was interviewed by the New Yorker magazine about the atomic bomb and related subjects 1945 Cassandra | The New Yorker
The interview starts:
““Astounding” has for the past 10 years or so predicted atomic bombs and using them to liven up its stories. The magazine got a notice of censorship on atomic energy, but Mr. Campbell wrote back that atomic bombs had been their stock in trade for years and it would look suspicious if they suddenly dropped the subject. The Army thought so, too. Mr. Campbell, a physicist and a former M. I. T. student, would like his publication to be taken seriously. His magazine is currently featuring a series of stories on post-atomic-bomb-war conditions. He discussed the next war and made some dire predictions.”
Unfortunately, I don’t have a subscription to the magazine so I can’t see the rest of the interview, but it certainly seems like a place Campbell would have boasted of his readership at Los Alamos or Oakridge if it were true.