Please don't get off the topic of what they were referring to.
What they were saying is that a two dimensional view of the bottom, top or any flat surface would not have the depth to see anything except a mark or discoloration. The tiles are as much as 4" thick. The pic will not show depth. They will see a mark(maybe) but not a gouge or to put another way, 3.75 inches or tile might be missing but they could never verify it from a two dimensional view.
Ever see a pic of the moon and think the craters are mounds instead of holes?