What does the attuned creature know about the Shield of the Hidden Lord?

The DMG states that all properties of a magic item are known to the attuned creature, or a creature that spends a short rest analyzing it, but what about the Shield of the Hidden Lord? Some properties of the shield aren’t usable by the attuned creature. Plus, it seems kinda boring to me that the player would instantly be aware of Gargauth’s true nature after an hour of attuning or analyzing. To me that is something to be found out via play or identify.