> explained that if breast cancer caused 3% of all female deaths and screenings reduced these deaths by 35%, that’s a good result on its own. But screenings may change mortality overall by only about 1%

This statement makes it clear what's going on. In ML terms: class imbalance. 99.9% of people won't ever get colon cancer, and therefore won't ever benefit from a colonoscopy. It won't make any statistical difference in overall population survival. But for the 0.1%? It will save their lives.