It's fine for Hanzo to know those spells! We don't require characters to use specialization slots for spells (or non-mage abilities for rogues or warriors, etc.) that aren't canonically specializations. The spec slots are only for things like shapeshifter, rift mage, knight-enchanter, etc., that are called specializations and have to be specifically chosen or unlocked in order for characters in the video game to use them, as opposed to the spell trees available by default to all characters. The one you linked is in the latter category, so you don't need to use a slot for Hanzo to know that magic. (Just make sure you take into account how out of practice and so on he might be.)