This image removes the bit of important context that they're talking about the safety of nuclear power. Elon is trying to argue that lingering radiation may not be as much of an issue as people may currently understand it to be. So, when talking about the bombings of Hiroshima and Nagaskai, he's trying to point out that even cities that were vaporized by an atomic blast can potentially be livable, and even thriving, within a human lifetime.

That being said, I would argue that there are far better ways to go about arguing in favour of the safety and efficacy of modern nuclear power plants without having to try to paint the horrific atrocity that was the atomic bombings of Hiroshima and Nagasaki, which have been reported to have killed approximately 200k people (civilians and military alike) in an instant and caused many thousands of later deaths due to burns and radiation poisoning and cancer and any number of other injuries caused by the blasts, as "not as bad as you think".

I'd argue that the great many strides that have been made in engineering and science to develop safe means of operating and maintaining nuclear power plants and the safe means of disposing and managing waste is a much better platform to start from for arguing in favour of nuclear power than the horrific accidents that have happened over the years which have themselves allowed us to gain the research necessary to ensure the current level of safety.

These are mistakes to be recognized, honoured, and learned from — not mistakes to be downplayed.