> Randomized colonoscopy screening has not shown a statistically significant all-cause mortality reduction.

My grandfather went to the doctor complaining of chest pains, they gave him a colonoscopy, and he died of a heart attack a week later! Clearly colonoscopy doesn't reduce mortality!

There's no reason for almost any medical intervention to have a statistically significant effect on all cause mortality. That doesn't mean it doesn't have any effect on mortality of individuals.