Yes they are. I even quoted the passage in a previous post. The
BRB tells us that all models either are units on their own or are grouped into units, that terrain pieces purchased for your army are models, and that those models are either Buildings (treated as a type of vehicle) or Battlefield Terrain (not treated as a type of vehicle). Fortifications purchase gor your army fill a battlefield role, the same as
HQs, Troops, Elites, Lords of War, etc.. They are treated as a specific type of unit that is factionless and in some cases can change sides during a game.
Actually reading the rules tends to answer most questions.
SJ