You're technically correct, there's no basis to argue that a "3D representation" was learned as opposed to "enough 2D projections to handle the inputs in question." That said, the hand which did not exist in either of the original 2D frames makes an appearance. I think calling wherever it was pulled out of "the 3rd dimension" is not wrong; it was occluded in both 2D inputs and functionally you had to know about its existence in the 3rd dimension to show it even if you technically did it by learning how pixels look across frames.

You can also see much more 3D ish things in the paper, with 2 angles of a room and video created moving the camera between them. Of course in some sense it adds to my point without detracting from yours...