Is a zone still considered "defending" if the opponent has not declared any defenders? Since the text on the BP announcement continues as "The latter ability, in particular, can force an opponent to keep developing zones, knowing that even the declaration of an attack could lead to a burned zone" I suppose it is...

It has to be. "When this unit attacks" is the trigger condition of the action. Since it is met outside of an action window, it has to be used at the first available opportunity, which is the action window after the declaration of attackers, before any defenders can be declared. I guess it would have been better to call it "the attacked zone" rather than "the defending zone".

Good point about the trigger condition, thanks for the comment.