The article covers it. If the 2013 Mickey is similar enough to Steamboat Willie to consider it the same character, then the 2013 version is a derivative work and does nothing to change Steamboat Willie entering the public domain. If it's substantially different enough to constitute a new character then it does nothing to change Steamboat Willie entering public domain. So either way it does not change what's happening in the new year, but Disney lawyers could argue that the new character is substantially different, so if you want to avoid getting sued base any of your works on the original Steamboat Willie and avoid any characteristics that make the 2013 version different.