It's not an on-off switch. Having smoked at all dramatically increases your chance of getting lung cancer. The more and the longer you smoke, the greater that chance increases. Quitting helps limit the increase in risk, but it doesn't eliminate it.
Why? What can possibly change at the cellular level to affect the body 25 years later? And how can they blame it on smoking and not, say, air pollution or Three Mile Island?
I'm not saying smoking doesn't cause it, I'm drilling down here.