I think when he says mirror finish he means in the mechanical sense, i.e. he's sanded the contact surfaces between block and back of the bridge down using progressively finer grades of sandpaper/belts/ to a flawless smooth finish creating 2 extremely flat surfaces increasing the amount of surface area contact between the 2 components.
Should, in theory, increase the efficiency of energy transfer between the 2, meaning no dampening effects etc.