That's a lazy assumption and it's wrong.
Discussion
Look at the video. They are wearing fine dresses and suits. How much manual labor are they doing?
People are killing themselves exercising when exercise was barely a thing back then.
> "Ultimately, the explanation here is that our metabolisms have been completely and utterly decimated over the past 100 years."
I think they didn't need to exercise to the extent we do today because they naturally got more exercise just in the course of everyday living. They walked more, had fewer labor-saving devices, did more by hand, and yes, industry wasn't yet sickening and killing them to the extent it seems to be today.