This is, of course, not the only problem with nuclear weapons (and by extension the science and technology that created them). First, there are several indirect ways that a single nuclear detonation could kill people. Second, the realization of nuclear weapons create a world in which nuclear war is possible -- one where mass detonations could alter the planet so radically that there would be innumerable indirect threats to everyone. Third, consider the way that the amassing of nuclear weapons so altered the geopolitical landscape, and what the effects were.
This is precisely what the letter-writer is hinting at -- that we should be competent enough to strongly consider possible unintended consequences. Despite the prominent stream of thought on this site, no particular technology is in itself "inevitable." We have the capability, collectively, to choose whether or not to pursue something.