standing or walking for more than six hours a day at work was associated with a doubled or tripled risk of needing surgery
Note that it's 6 hours of standing! Also, important to remember that these are correlation studies meaning that surveyors (I refuse to call them "scientists") sends outs surveys and tries to find statistical correlation. This is much different than real scientific studies, i.e., causal studies, where you try to understand cause and effect of your hypothesis.
One of the major issue in sitting all day is pasture management which causes back pain issues. So it's not about getting cardio exercise (that would be silly) but avoiding long term back pain issues. Also, sit-and-stand desk allows you to adjust height easily which is again important through out the day.