if a Mage of Void knows that which is obscured or unknown would they basically know everything that isn't known? same for a Seer of Void

Yes, and no. As powerful as the potential of “Knowing the Unknown” may sound, remember there are Rungs of God Tier powers, and different people will use it in different ways. So maybe they do acquire knowledge of EVERYTHING that is secret and unknown! But maybe they can only decipher code if they’re actively reading it rather than just suddenly knowing it, or maybe as the “Knowing Nothing” also implies, they could have absolutely no idea of anything that’s going on. You can definitely toy with God Tier powers and what degree of power and what parts of the Aspect they touch.