No, it appears not to be. I just located the new dungeon, and it isn't located on the last bit (west) of the whole Papaya Isle complex, but instead it's located on the southern tip by the tiny isles of Papaya Main Island. In fact, the Turtle Island part is still sealed off with another UD to come in the future.
The new dungeon have a start quest (and possibly a whole quest chain to it) -started by accepting the quest from the Police Man at the third teleport.
Here are 3 photos of the location + how the inside looks like:
Quest accepted from the third teleport, and quest objective at the entrance of the new UD.
Clear view of the Coral Cave Dungeon entrance.
Sneak-peak of inside beginning of the UD.
632
