Isn't this one of those standard logic fallacies? I mean, A is true, B is true, but that does not mean B is true because A is true? There might be plenty of other factors affecting B. The advance in medical science for one. At least I always thought that alone was the most influencial factor on better life expectancy. Probably more than enough to even undo the effect of certain lifestyle habits making things worse.
But yes, the article pretty much fails to provide exact numbers or even estimates on how bad a sedentary lifestyle really is.
Instead of assessing the problem as "sitting," maybe we should look at what scientific data says. I know food manufacturers put a hell of a lot of sugar in foods, even foods you wouldn't expect, like spaghetti sauce. They use it as a preservative, so it's in damn near everything.
http://www.bbc.co.uk/guides/z8849j6
I know watching food commercials subconsciously compels people to eat, even after the restaurants are closed. Children consume 45% more food when exposed to food commercials.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2743554/
I know certain foods, sugar in particular, have addictive properties similar to cocaine.
https://www.ncbi.nlm.nih.gov/pubmed/23719144
Sitting may very well be a factor, but this article shows no links to any studies. Until I see some data, I'll consider it BS and so should you.