That outcome would be expected from either decreasing cell size, or increasing bits/cell (what used to be commonly called SLC/MLC/TLC/QLC).

I don't see TFA mentioning either directly, but there are some hints that one of the two (or both) did happen: Number of layers increased from 176 to 232 (that explains 32% increase) but bits/chip area has doubled (total density increased by 100%). Where does the rest of this increase coming from, you reckon?