The article needs to be taken with a grain of salt.
Our infant mortality has gone down, leading to many more "weaker" individuals making it past five (of course a positive thing for most). These individuals are likely to have a shorter lifetime expectancy (sometimes far too short), dragging down the average.
These individuals are sorted out entirely in the historic data, as they would not have made it till 5 years of age back then, which is the age they use for the comparison.
In other words, it does not produce any evidence that a healthy individual from back then is any healthier than a healthy individual now. It merely shows that our weak individuals live longer than back then, turning an infant death into a maybe decent life.