But there’s another part of both the Franck report and the Szilard Petition: They were concerned that once nuclear weapons were used, it was inevitable that other nations would develop them, e.g. this part:

If after the war a situation is allowed to develop in the world which permits rival powers to be in uncontrolled possession of these new means of destruction, the cities of the United States as well as the cities of other nations will be in continuous danger of sudden annihilation.

That is mostly a sentiment repeated from the original Einstein–Szilárd letter.

Where I would have really appreciated the author go is that Szilárd is likely the cause for nuclear weapons to exist as we know them. He presumed that Nazi Germany was developing nuclear weapons and took actions to encourage the USA to create them first. History shows us that Nazi's weren't as smart and instead went down the path of developing their version of the technology for nuclear power plants, not bombs.

There was no risk of the powers of WWII of either side building atomic bombs until Szilárd's actions put us on a course for their creation.