In-game I'm still a little troubled about that. Because, if neither is Dracula's Castle, how can Death summon the true one from two fake ones?
He also says that "[Maxim] brought this castle into existence once again." If this is not Dracula's Castle, then it's not "once again."
How can Death summon the true castle from 2 fake ones? The way I see it is more like this in a nutshell:
- Castle A; brought about by Maxim (A) who is the regular Maxim and also this castle's final boss. Fighting and killing him delivers the bad ending.
- Castle B; brought about by Maxim (B) who is the dark Maxim* and also this castle's final boss. Fighting and defeating him delivers the preferable ending. Prior to this ending Death informed Juste the Castles were already merging.
*Dark Maxim being a manifestation/entity of regular Maxim's darkest desires which came from the influence of Dracula's remains on Maxim("Dracula's power was overwhelming"; Maximto Juste)
- Castle A+B Merge; The outcome which happens when either:
1) Lydie becomes the sacrifice and vessel for Dracula's Revival, which would have resurrected the real Dracula (not "Dracula Wraith"). It is a possibility the true Castlevania would result from the Dracula's resurgence.
Or
2) Dark Maxim merges with regular Maxim (no longer being a manifestation of his darkest desires) making Maxim the suitable vessel for Dracula's resurrection (Not "Dracula Wraith"). This is not as favourable an outcome as (1), but it's still possible for Dracula to be resurrected and the 2 castles to merge this way.
The reason why (1) and two never happened were because Juste got there in time to save Lydie, and he got to Castle B's final room after gathering all the remains in time to stop Dark Maxim/ Regular Maxim and Castles A/B from merging.
It was never the true Castle in-game, Death just assumed that Juste would never make it in time to stop outcome (2) from occurring.
If either outcomes 1 or 2 would have happened, then it would have become Dracula's castle. Perhaps this is why Iga as said something to the effect of "it may not be Dracula's Castle" or something to that effect.