That's not exactly what he said, and the distinction may be important. He says, "the little dimples in the rough side create more cooking area because the hot air gets trapped inside there and it gives it a really good looking crust."
I'm not exactly sure what that means by more cooking area. The cooking area is the size of the stone, I think? If he's talking about contact area, it seems like if anything it would be the opposite. Air is orders of magnitude less conductive than stone, and roughness would have the effect of lowering surface contact and therefore the average conductivity. Maybe intermittent surface contact from a rough surface is better. I don't know, but his explanation doesn't make sense as to why it matters.