Well, I have to admit this one was a surprise to me...
As of V2.0 of the rules, Assault as well as Firing can trigger the Destruction ability. It was likely an error that the change was left out of the first version of the SoN rulebook.
So in the Compendium version, the text states:
If a structure or a unit inside a structure is targeted by gear or a unit that has this special ability during a firing action or an assault, the building may suffer destruction points.