No doubt sitting is bad, but don't overthink this. The original and following studies could not possibly have controlled for every related variable. There's just too many potential nodes and arcs.

If you're in generally good health, get a standing desk and call it a day.

It could also be argued that having a treadmill desk leads to spending more time at the desk. Since treadmill means less work done due to fatigue + less productivity because of loss of deep focus. Overall, bad news. Standing desk + general cardio is what I recommend.