I don't think that it's sitting that kills. I think it's more the fact that you are less active. You can have the same problem standing all day without moving around.
So either sitting kills by itself and standing up is enough to avoid it, or sitting kills by taking time from being physically active and it's not enough to stand up (which also takes time from being physically active).
Sedentary behaviour, physical activity, and risk of coronary artery disease and diabetes mellitus
"From the accelerometer-derived parameters of SB [sedentary behaviour], a higher number of daily SB bouts lasting over 10 minutes was the strongest predictor of a high CVD [cardiovascular disease] risk."
