Activated Defense in Assault

[Schoenwulf]

Rule 12.4 indicates " Note that defending units do not have to activate to fire, and are not marked with a Moved/Fired marker at that time. " when they are assaulted by an adjacent attacker. However, if a defending unit has already taken its activation, either via "move" or "fire" actions in the round in which it is assaulted, is it still able to return fire in defense? It would seem so since activation is not necessary for an inactivated unit in assault, but the rules  imply that a unit activates once per round. Ergo, is defending an assault not considered an activation under any circumstances?