That quote is taken from the Sandtrap retrospective and it seems that, once upon time, a multi-tiered approach was considered for one of Halo 3's iconic maps.
This ideas for Sandtrap is the idea of having a smaller area locked away from the player, but when the play makes it to the top, they gain access to the bowl and an area about the same size as the place they came from.
Whilst obviously the implementation of this approach is slightly different from how PDZ managed it, I think it would have been a novel idea if Sandtrap had continued along those lines. It's interesting to note, from my experience with the Battlefield Bad Company 2 Beta, it had multi-tiered levels, players would complete an objective or reach a specific amount of kills in one area and the next area would unlock and the defending or loosing team would spawn in the newer area and set up camp as the other team marched towards them. Again, it's not the exact same idea but a different approach to the same kind of design.
I'd love to see Bungie take on a multi-tiered level design, if only to see how differently they would approach the concept. Would they revisit the old Sandbox design or would they start from scratch with an original take? Bungie have taken older ideas from the cutting room floor (Forge, Firefight and Saved Films notably) and implemented when they felt they could and when you watch the march multiplayer trailer, it isn't too far-fetched to imagine something different.
Whilst I don't think it's likely that we'll see such levels, I'd be happy to be wrong.