Double jointed people can do that. The only question is can they do that with enough finger strength to counteract the double jointed thumb pushing from the opposite direction?
In this picture, the wall is pushing the fingers into that position. And I don't think this hand could hold a soda can, even though it can be put into that position.
But in this picture the fingers are doing it under their own power. Are they doing it with enough strength to oppose a thumb? Maybe. Maybe not.
