Have you never worn an air-boot before? If you have a minor fracture, something that doesn't require a full immobilizing cast to heal, you get an air-boot. You can still put pressure on it. I suffered an avulsion fracture a few years ago and wore one of those boots, and was still able to put pressure on it. He's clearly not standing there on one leg for an extended period of time, he is walking and the picture is a freeze-frame to stir up a conspiracy.
Remember when W Bush was bruised from choking on a pretzel and fainting? Biden actually got a bruise back when he was VP from playing with his dog. Strange things can happen.