Uhm... ...isn't this something that needs to be triggered, hence not making it passive?
- Guardian, the arte, is an arte. As an arte, it can be set to the arte buttons like any other. It's considered an arcane arte in Abyss and Symphonia only. Symphonia also has those other differently-named arcane artes that do the exact same thing for different chars, but they're artes too.
- Magic Guard, the passive skill, can be accessed only by doing the square+down command. This exists in almost every game since Destiny 2. Abyss allows Guardian to be accessed instead of Magic Guard this way, once Luke/Asch/Guy learns a specific AD-skill. But Guardian itself is still an arte.