Maybe the article was updated since you read it? It goes into their methodology: "Arias and her colleagues calculated life expectancy using a technique called

Maybe the article was updated since you read it? It goes into their methodology: "Arias and her colleagues calculated life expectancy using a technique called a period life table. This involved the researchers imagining a group of 100,000 hypothetical infants and applying the death rates observed for the real population in 2021 for each year of those infants’ lives. The result is not the life expectancy for a cohort of actual babies born in 2021 but rather a snapshot of how life expectancy rates would apply to various age groups at a specific point in time, Arias says."