Yes, I understand that. However, the particular leaflet cited was not, as far as I can determine, dropped right before the atomic bomb, but before conventional bombs shortly before that. And the Japanese were becoming immune to the messages falling from the skies. Also, the leaflet did not warn them of any unusual weapon, so even if it had been dropped right before the atomic bomb, its message was basically "here's more of the same old thing."
That said, a warning of a fantastic new bomb would not have impressed a lot of people at that point.