Pro or con the diet, the difference in life expectancy is mostly antibiotics.
Life expectancy made huge gains throughout the century before antibiotics.
You can't really see the invention of antibiotics clearly in the curves.
The way I think of it is that of course penicillin increased life spans, but the discovery and industrialization of it was highly correlated with rising GDP.
I share the feeling that better nutrition is probably not as much of a significant factor in developed countries. For what it's worth, we are more obese than our ancestors.
Our World in Data has this nice chart of life expectance by age in England and Wales [1]. I have this pet hypothesis that a good chunk of the increase in the life expectancy of the younger people in that chart is due to vaccination and better hygiene. And, for older ages, the a good chunk of increase is due to advances in surgery.
The effect of antibiotics is probably more uniform across the ages. It is note-worthy that penicillin was discovered in 1928, so the increase in life expectancy before then can't be due to antibiotics.
I have no expertise in this area, so you should take all of this with a pinch of salt. The Our World in Data chart is really cool though. That steep fall and rebounce due to the Spanish flu, and the contrast with COVID-19 with respect to affect on different age-groups is also interesting. [2]
[1]: https://ourworldindata.org/life-expectancy#it-is-not-only-ab...
[2]: I think comparing the naming of the diseases also tells us something about how society has changed.