3d cells for nand aren't the same as 3d CPU cores. Nand is incredibly simple which makes it possible to do things like 3d cells which would otherwise be more complicated.

There's a reason memory can generally beat computational cores to smaller node sizes. It's because highly uniform circuits are much easier to manufacture. A lot of problems present in CPU logic cores (such as cross talk) just aren't present to the same extent in NAND cells.