How does LIDAR avoid getting confused in an environment being scanned by hundreds of other LIDAR?
Commercial LIDAR units modulate the light with a very long pseudo-random sequence. 

The modulation is primarily to (1) have a modulation for determining distance and (2) to avoid interference with ambient sources of DC and AC light. 

The long sequence makes it unlikely that any other source, even a modulated one like another LIDAR, will line up and interfere.