I think your test is biased by the fact that the test panel is arched on a rather tight radius. That's an ideal shape for stiffness and it transfers it's load well. With your setup a lot of the loads are compressive.
On a hull the 4 oz. interior glass would be subjected to a severe trensile (stretching) load if the hull was hit from the outside. Those kinds of loads are most common. The exterior glass would be subjected to tensile loads if a force capable of bending the hull was applied. Because of the tubular shape of hardbody kayaks they usually have enough stiffness to tolerate ordinary bending loads.
I have an older design kayak made from 3mm plywood with no glass sheathing in or out. It is extremely stiff because it is extremely tubular but it's ability to ward of pentration is not it's strong point. That's the situation you will be facing with very light glass sheathing.