From the article "The idea is that all else being equal—same recipe, same oven temperature—because of its superior thermal qualities (higher volumetric heat cap

From the article "The idea is that all else being equal—same recipe, same oven temperature—because of its superior thermal qualities (higher volumetric heat capacity, as well as higher conductivity than stone), you can cook pizzas faster than you'd be able to with a regular stone." So it is thermal capacity plus conductivity together.