He’s talking about the pillar debris which has the exact appearance of stone and rock whereas the ship’s outside (which is what took the 6-A landing) is explicitly
drawn,
bends, and
breaks like metal. This is
shown numerous times. There is a difference between the interior and exterior of the ship and this is even further emphasized when once the ship falls,
you see stone, not metal falling from above in Boros' room. Stone with the same appearance as the stones from the
pillars and
door in that room.
And very solid construction doesn't mean much. The rooms in a house can have very solid walls but that doesn't mean the walls that separate the room spaces are made out of brick just because the outside walls are.