The description just says "The skill bonus of the Defender is added to his DB versus melee attacks. Against missile attacks the skill bonus is halved and added to the Defensive Bonus."
I read that as applying against all attacks, with the stipulation (as mentioned above) that you need to be aware that you are under attack. Parrying is covered in 23.2.6, and Adrenal Defense has its own heading 23.2.8, so AD isn't a type of parrying and I don't think the restrictions on parrying apply (e.g., you can only parry the foe you are attacking).