The tiles are extremely low-density, so their heat dissipates quickly because they don't hold much of it. So the corners of the tiles become cool within a few

The tiles are extremely low-density, so their heat dissipates quickly because they don't hold much of it. So the corners of the tiles become cool within a few seconds, even though the material inside just a centimeter away is at thousands of degrees. That's why he said to touch the corners. And why you can see the corners not glowing, while the center of the sides keeps glowing.