As you said, currently, this trait is attached to the 2 Industrial Era artillery line units, Howitzer and Siege Artillery. Judging from the Bombard trait description, I think they can attack both Districts and Enemy Units. "Can bombard battlefield from outside the battle" probably indicates that Bombard units can participate in Tactical Map Battles, so they shall able to attack units as well. For Contemporary Era, interestingly, Humankind doesn't have any generic modern artillery units - there is a modern EU, very likely a rocket artillery for China - but no generic modern artillery. I guess this is something being overlooked by the devs, and hope they can add in some modern artilleries in the future.