Then Genocide City was more completed (because this is a level with a slot in the game) than the sand/snow zone (wich had no slot).
I highly doubt that personnaly.
GCZ has a slot under the level select, like all the levels we know, except the sand/snow level.
If GCZ had nothing made (even some art):
Why make a slot for an empty level, and not for the sand/snow zone with some art already made?
If GCZ had already some art somewhere:
Why keep an empty slot in the level select for something cutted from the game very early, like the sand/snow zone?
Conclusion, 3 things possible:
1. The sand/snow level may never existed (even some art). Brenda Ross was lying under the interview, and the mockup is a fan art picture.
2. The sand/snow level is Genocide City zone.
3. The sand/snow level existed, but was replaced by another level using the same slot.
That's what I think.
There is something always true: Dust Hill zone is Mystic Cave zone in OUR Sonic 2 beta v1.54.
Maybe this is not true in older versions of the game.