I probably overlooked this in the article, but areas of high elevation probably are sparsely populated, and this means less sampling. Less sampling means greater variance. This correlation could actually be due to the variance in the sampling. IE at some later year, we'd find that the regions of high elevation actually have increased rates of cancer. There are several known cases of reported correlations being due to nothing other than small sample sizes, like when the Gates foundation donated millions of dollars towards an effort to break larger schools into smaller schools after finding that smaller schools were more correlated with good grades. Turned out that the correlation was only due to the fact that smaller schools have less samples sizes.