Does anyone know why this is the case? Do mirrored surfaces trigger some sort of internal reflection which ends up absorbing more energy?
The SR71 was famously painted black as a mitigation for surface heating issues because the black paint conducted heat away from the areas the were really susceptible to ram-heating.
Perhaps reflective surfaces reflect some percentage of the incoming energy away, but thermally conductive surfaces conduct a larger percentage of that energy away and are able to safely sink it into some thermal mass.
I expect the counter will be more along the lines of having sufficient ablating material to block the laser - a small amount of extra armour probably has an outsize affect on survivability.