I think the buildings all got erased by the mudflow. See how the multiple storey carpark instantly burst and collapsed when hit by the mudflow. The mudflow was not just water and mud, had countless gravels, numerous huge boulders. See the france news video showed how the mudflow travelled very far downstream still had energy to roll huge boulder.
The gyirong port is so close to ground zero, Just one of the huge boulder is enough to demolish the buildings. The countless gravel are like bullets will erode the buildings grain by grain.