No, that is not correct.
At school a long time ago I saw quite a few pictures of the post-blast Hiroshima and Nagasaki (the presentation was considered sufficiently graphic that parents had to sign a consent for a kid to see it)
As I recall, some of the shadows were light, some were dark.
For example, if there was a white/light grey concrete wall painted a darker color and someone was standing in front of it, the blast might burn away the darker paint except behind the person, leaving a dark shadow.
Conversely, if the blast charred a surface, except where a person was standing in front of it, then the shadow might be lighter in color.
The shadows are where a person or object protected something else from the full force of the atomic blast, that’s all. It the difference between the blasted surface and the somewhat protected surface that makes the image.
Any ashes or soot generated, however, would have been blown from the area by the massive air movements caused by the blast, ditto for ay survivig body parts. The shadows are images, they aren’t actual human remains in the sense of being bits of burned body.