Thats correct, its a monocoque body and only outer skin and floor plans are lightly armoured, majority of the body is 8 or 10mm with 12 and 14mm (the original armoured plate was metric) in some places like the front panel, roof hatches and front turret. All the internal panels and gussets are just thin steel 1.2 - 1.6mm.
Its a very agricultural design compared to the Humbers and Daimlers etc, ideal for Nuffield to build :-)