I see that and I'm glad they included it, but ...

Was 1999-2000 that golden age when we all threshed the fields by hand and steered the plows on foot ?

I remember people being quite sedentary and obese then and I don't know that an activity comparison to 1999-2000 has any utility ...

EDIT: OK, I am thinking about this backwards - if, as I suggest, people were just as sedentary in the first period as they are in the second, then the T drop begs a different explanation. As you suggest, they also corrected for age.

So, perhaps, my instinct is wrong. I would still look very closely at the (adjusted, corrected) physical activity before drawing any other conclusions ...