I thought the event was accessible as long as you had an event Arceus (although you could also use an Arceus from the DPP in-game Arceus event).

The unused one is from DPPt, just play the Azure Flute and Arceus will be in the top of the Hall of Origin, inside the Spear Pillar (with a very LONG stairs)
It was scrapped because they thought "it too hard for kids to understand"