In strategy mode.
A little more testing, this howitzer does massive damage to 3 Warlus in a base roughly 600 meters away. It also works on the turret, but not the garage.
The shell cannon on Walrus also does no damage to buildings without direct LOS.
and yes like MadDogX said, the shell cannon on carrier is working fine against both ground units and buildings.
Probably only those can be targeted could be destroyed by long range howitzer?