I'm guessing when they say standing they mean standing still with limited/no movement? Perhaps demands on the heart are increased standing still vs sitting? From memory doesn't the soleus(and likely other muscles) aid in venous flow? But this would require movement, so unlikely to happen standing still.
I think they meant specifically those standing desks where you stand in front of them more or less still for hours at a time. Come to think of it, it is pretty obvious that this would be more stressful/bad than sitting. Who stands still for hours in the same position? The core muscles would get pretty tired at some point and with no movement the blood may begin to form clots. The incidence of varicose veins is highest in professions with prolonged standing, even if they more. Waiters, guards, concierge, etc. It is not a coincidence.