This is 321 physical layers of silicon in an IC, not 321 charge levels.

QLC flash - with 16 charge levels, for four bits per cell - is pretty common nowadays, but that's as far as it goes so far. And stability is indeed a concern; modern flash devices rely heavily on error correction.

Memory like HBM uses stacking of silicon layers, but NAND flash isn't layers of silicon, it's other materials for capturing and holding charge with still just one silicon plane it's all built on top of I believe.