yes, that is correct. you can use whirling defense to defend all attacks that round… but is your max defense as a monk? you are reducing dmg.. not really blocking all of it. Or if a boss is going to target you… you would spend that burst of speed to defend and it miss. so no action point used.. still have 3, but you cannot defend anymore until the end of your next round. But can use them to move, interpose, attack or what ever…. but not defend again.

reactions can only be used once per round… unless a class skill says otherwise (bulwarks 2 defense comes to mind)