Not to say that people didn’t realize that radiation could be dangerous, it just wasn’t widely thought to be all that dangerous, and it wasn’t really publicized how easily it could cause injury.
This patient is showing moist desquamation, which is fairly common in incidents of acute mid-level radiation exposure. Cell damage is pretty deep in injuries like this, and there’s usually some degree of radiation fibrosis when the site heals.
A Treatise on Diseases of the Skin for Advanced Students and Practitioners. Henry W. Stelwagon, 1923.