The article handwaves working patterns away with one sentence: "Many of them were farmers or laborers, of course, but plenty of people in 1900 had cushy desk jobs, and those people weren’t obese either."

I can't find good numbers on it, but 3-5% had a desk job in the year 1900, seems like there was an entire article's worth of points here but the author wasn't interested which is a shame. At the very least there's definitely more to debate there than with the altitude example.