You're implying that accounting for the fact that people in Denver get "2x to 4x the average radiation" but "have far lower rates of cancer" is a problem for people who saying that small multiples of radiation exposure raise cancer risk, but ignoring that a lower incidence of cancer for Denver would have to be accounted for in any case, no matter what you believe about the relationship between radiation exposure and cancer.

Unless your actual argument here is that radiation prevents cancer - and I'd say that you have a higher burden of proof for that.