There is a difference in where the mutations occur in the body. Sunburn still primarily causes mutation in and near the skin. radiation can cause mutations in the organs deeper in the body which are less robust against radiation, for the exact reason that the skin is usually a sufficient barrier.

I mean, yes, sunburn is excess ultraviolet radiation, but there are lots of other (more severe) forms and it’s pretty straightforward to understand they mean “excess [beyond the type experienced in the comparison populations, namely, the general population, who tend to experience minor exposure to excessive ultraviolet radiation, aka sunburn].”

This isn’t the “gotcha” you think it is.