That Belmont Club article explains that the enemy in Fallujah is now mainly holed up in a neighborhood called the "Golan": a labyrinth of lightly constructed buildings in which those familiar with the territory can easily hide. The walls and roofs of these buildings can easily be pierced by even a modest-sized weapon, allowing either side to shoot right through buildings. There's good news for our side, though: the task of pinpointing the enemy may be formidable but not insurmountable. "The battle for this urban maze will be largely a battle for line of sight as it probably has been from the beginning", Wretchard explains. Our night-vision capabilities are much better than theirs, and scouts and snipers have been patiently watching the area. Also, captured enemy fighters may provide useful intel.
The flimsy buildings of "Golan" will provide the enemy some protection from observation, but none at all from weapons fire. Once the enemy's position is known, the article concludes, he is finished.