Real-life anachronism?

I am confused by some in the first segment, "Comrades" (starting at 27:47), where the Big Three are sitting in chairs facing an invisible audience. These scenes are so replicated in still photographs. The confusing part is that FDR can be seen crossing his legs, and even bobbing his foot up and down. This appears to directly conflict with the notion that FDR suffered "complete paralysis" after 1921-- well before his meeting with Stalin and Churchill.

I've re-watched the "footage" and it is evident to me, especially because of Roosevelt's nonchalance, that his motions are both deliberate and he has feeling in at least his right knee and foot. Can someone help explain this to me? It seems like I'm missing a huge piece of the puzzle.

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