If that F-350 can tow 21,600 lbs then it must have a GCWR of at least 28k-34k lbs, unless they limit it to 26,000lbs exactly and that truck weighs less then 4,400lbs which I highly doubt (my 1/2 ton weighs 5,200lbs). If that truck has a GCWR of 26,001 lbs or more then you need a CDL license to drive it, even empty.
Section 259:12-e Commercial Motor Vehicle.
Ford is just playing the numbers game as usual knowing most people don't have a clue about GCWR's and GVWR's and all they see is a tow capacity higher then another truck so it must be better.
The Ford truck's that are rated at 26k GCWR or less so you do not need a CDL are only rated to tow 18k lbs which is around the same as the others. In order to get that higher 21k tow capacity you have to move up to a truck with a 4.30 rear end and a GCWR of 33k lbs which is way over CDL. So for this new F-350 to have a tow capacity of 21,600lbs, I guarantee it will have a GCWR of over 26k. This is for the 2010 models but I doubt much has changed for the 2011's.
2010 Ford Super Duty Towing Capacity | The Official Site of the 2010 Ford Super Duty | FordVehicles.com
Did you look at the definition of "GCWR"?