I don't think you're correct, although I upvoted you before I thought that because you seemed to be explaining the problem I had with the article. However, reading the article's comments it becomes clear that there's 81 locked boxes, one for every cell in the Sudoku grid. The key Victor chooses from the 28 Paula proffers will open nine of the 81 boxes. Those nine have to be consistent amongst themselves, i.e. a permutation of 1-9, despite Paula not know which of the 28 keys would be chosen.

Charlie doesn't exist, nor does he need to.