Isn't a 6x6 without the front wheels driving be 6x4?
No not realy, its a rear wheel drive vehicle. The term 4x4 and 6x6 only applies to the design capability of the vehicle or the number of axles it has, all wheel drive etc and not which wheels are operating at any given time.
How is the rear 4 wheels driving considered 2 wd?
I suppose they could have been more clear by saying 'rear wheel drive' but they were trying to explain that the front wheels are not
pulling.
Fwiw - The term 4x4, 6x6 etc came from the military during WWII in which the first number indicated the number of wheels and the second number the number of driven wheels. It was to avoid confusing as to which vehicles might be 4x4 etc it was wight on a metal plate on the dash.