The only redundant part of this snapshot is the second 512k^2 snapshot of the overworld. The End and Nether snapshots are still meaningful. Excluding the 512k^2 snapshot, the size would be around 12TB.

And the actual size of the 2b2t world is likely much larger than 15TB. The data for this project is stored in a highly compressed form, much more efficient than the game's standard file format.

Microsoft can probably scale Minecraft up to support infinitely large sized worlds if they want to. Minecraft lazy loads the world in chunks so you there's no real limitation (aside from machine digit sizes)