That's not necessarily a disproof - if opioids are killing people who are disproportionately young, then they could have an outsize effect on life expectancy. Say life expectancy is 79 years. If a new cause of death occurs that affects 10% of the population and kills, on average, at age 78, then life expectancy goes down to 78.9. If a new cause of death occurs that affects 10% of the population and kills at age 29, then life expectancy goes down to 74. Same reason that infant mortality had a disproportionate affect on life expectancy: the numbers of deaths might be small, but the reduction in lifespan is large.

Personally I would bet on obesity being a bigger contributor to declining lifespans than opioids, but I'm just showing that raw numbers of deaths cannot automatically disprove the hypothesis that a decline in lifespan is due to opioids.