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.
There are no hidden rules or rules specific to the Shield of the Hidden Lord. An attuned creature knows as much about the Shield of the Hidden Lord as it would any other magic item.
You already know the answer and have stated it in your first sentence though there does seem to be a slight misconception.
Attuning to a magic item doesn't give you knowledge of the item's properties, it just gives you "an intuitive understanding of how to activate any magical properties of the item, including any necessary command words".
What those properties are, exactly, still requires identifying the magic item which can be done during a short rest or with the Identify spell.
The DMG also specifically has an optional rule if you think that a short rest is "too easy":
Variant: More Difficult Identification
If you prefer magic items to have a greater mystique, consider removing the ability to identify the properties of a magic item during a short rest, and require the identify spell, experimentation, or both to reveal what a magic item does.