Because not all ankle injuries are the same. I don't remember the name but that sports medicine doctor on YouTube gave a thorough explanation of how it's entirely plausible for him to be fine to do jumping jacks and whatnot, but actual on field action would be too painful. Stuff to do with which tendons are damaged, how different actions flex the foot in different directions, and where the bone spurs on his foot would be.
I'm going to take his word on how things affect athletes over random people on the internet going "pfft, I saw him jog through the endzone so I think he's faking it".
*edit*
Found the link.
https://youtu.be/TSYeQkRPkNQ