[Someone]

I don’t see how that says much about the usefulness of colonoscopy.

The people in the treatment group who didn’t show up must have had a 36% increase in colon cancer death (not explicitly stated in the article, but can be derived from the numbers. You need 42% × 50% + 58% × ? = 1), and (solving 42% × 70% + 58% × ? = 1) a 21% increase in colon cancer risk.

Something must have made them different from the control group. Maybe, they didn’t show up because they already were being treated for colon cancer?