I still don’t understand what the impact of the name alone would be. Like unless it’s a name that’s known in the verse, what’s the point of hiding it from the audience?
I've come to the conclusion that the name of the Ancient Kingdom has to be some myth that is known worldwide, like the One Piece is known in the modern era.
The name of the Ancient Kingdom would be a name everyone in the world knows, but everyone believes to be a myth, like the Sky Islands/The City of Gold
I am someone who believes the theory that Skypiea is on some levels a condensed version of One Piece's overall narrative. So, to me, this scene more or less confirms the Emerald City exists in One Piece.
Do I think the Emerald City is the true name of the Ancient Kingdom? No, I think it was a city, possibly a capitol/the capitol of the Ancient Kingdom. A city, like Shandora.
So, the Ancient Kingdom would be connected to The Emerald City.
If we're talking about sunken cities connected to Emerald, then, I believe Oda is refencing the Emerald Tablet of the Atlantean Thoth, which... is about how everything is connected to light, more or less. IE, Luffy, the Sun God.
I'm not sure what the name of the Ancient Kingdom is, but, I do believe we can confirm the Ancient Kingdom is directly inspired by the legendary kingdom of Atlantis, which was said to span the entire planet. They City of Gold was in the sky, so, I expect Emerald City to be... underwater.
So, the name of the Ancient City is the One Piece world's equivalent of Atlantis.
It would be like Vegapunk saying accepted history is fake and a fairy tale is real.