Unfortunately the reality is worse. Human vaporization isn't really a thing. You can get blown apart by the shockwave, but even with the gamma radiation causing thermal waves of 10,000 F it still won't instantly incinerate a human. They just get flash cooked.

The shadows are caused by the person absorbing the intense light and thermal radiation emitted by the blast, which peels material off the building surface.

If they were close enough to the blast to be instantly killed by force or heat, it would most likely have been a strong enough force to also knock down the wooden building the shadow was etched into, or set it alight so it burned down.

It's unlikely many people caught like this lived long, but sadly it's not a given that they all died instantly or painlessly.

https://www.livescience.com/nuclear-bomb-wwii-shadows.html

https://en.wikipedia.org/wiki/Human_Shadow_Etched_in_Stone

Getting old ain't for sissies.

That was a very interesting read. Thanks for the link.