Doesn't that require assuming the conclusion? If LNT is wrong then skin cancer shouldn't be linearly proportional to radiation exposure either.

Plus, you still have the same problem with confounders. Expect more skin cancer in a population that spends more time outside, but also potentially a very different lifestyle in that case.

And problems with radiation type as well, since many types of radiation don't penetrate the skin, so they could cause skin cancer but not other cancers. Which has implications for the fatality rate because skin cancer has a much higher survival rate than most other types.

You’re always stuck building a model and testing it in any case. The point is you don’t need to adjust for many common causes of cancer like smoking, so it’s a simpler model.