As a person with some background in metal fabrication, I’ve taken casual notice of the fabrication, assembly, and materials used in these machines. I didn’t need to take one out to the desert to determine that they are highly bullet-resistant just by their design and materials. The testing chart in your OP certainly validates my casual observation.
They do seem to be constructed out of steel that is much harder than mild steel, I’d say it approaches armored/ballistic steel. But the sides are like 1/4”, so it’s not stopping a .50 cal any day.