Because it is a truism. If they lived as long without cancer as everyone else, they per definition are not more susceptible. It's simply a restating of what being more susceptible implies.

> If your “hypothesis” were correct, the rate of death would decline after the early fatalities occurred.

This does not logically follow. This assumes that removing people with a genetic or other predisposition for getting cancer early would reduce the average risk of cancer more than increasing age increases it.

Mortality rates compounds. Reducing your average risk of dying of a given cause by even a tiny proportion on a per-year basis adds up to substantial chances of surviving longer.