Indeed, what is it? The article doesn't say, only espouses the supposed benefits.
https://wccftech.com/intel-showcases-its-zam-memory-prototyp...
The connectors on the side indeed look like the letter Z. Maybe it disperses the stronger currents across the stack of the crystals, instead of concentrating.
And why it's not currently done is likely because it's hard enough to stack when everything is uniform. A small deformity in the first layer will spoil the entire chip.