The chocolate brown burning super-plastic base clay (to which 20% coarse grog is added) matures at cone 6. Yet this is a cone 10R body. The grog stabilizes the fired matrix enough that it stands up in the kiln. And it fires to a dense product that can withstand any weather. Any porosity that can be measured is only from the grog. A number of manufacturers around the world make bodies like this, some can have almost double the grog this one has. These employ engobes (a brown and a blue, applied at the leather hard stage - L3954N) on the insides, enabling a smoother glaze surface.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.