That leaflet specifically mentions the destruction of Hiroshima. It was a wide spread fear campaign AFTER they dropped the first bomb. There was no forewarning.
Looks like you might be referencing the second leaflet mentioned in that article.
In August 1945, leaflets were dropped on several Japanese cities (including, supposedly, Hiroshima and Nagasaki). The first round, known as the “LeMay leaflets,” were distributed before the bombing of Hiroshima. These leaflets did not directly reference the atomic bomb, and it is unclear whether they were used to warn citizens of Hiroshima and Nagasaki specifically.
Seems like there is some ambiguity though, since it says they were supposedly dropped on Hiroshima and Nagasaki but then says it was unclear.
