Yes, here "overall" is being used as a synonym for "from all causes" not "over the full length of your life," and there's this frustrating thing for those of us with physics educations where people frequently leave off a unit of time: so like if it's a 5 year study, and your all-cause mortality is lower by a factor of 20% over those 5 years, that means 80% survived and you should compute the fifth root 0.8^(1/5) to find that 95.6% survived per year on this account, and then you can say "reduces mortality by 4.4% per year," and that sounds much more reasonable.

This happens all over the place. You're just supposed to know in investment that a price-to-earnings ratio is measured in years, or people will say "the Buffett indicator is 200%" not "the Buffett indicator is 2 years."