nm is not referring to the die size!
"We don't know exactly how small the NAND dies are going to get"
The dies itself doesn't shrink too much... It's the stuctures (transistors, etc.) that shrink. More tiny the structures -> more of them can be fitted on the more-or-less the same sized die. It's the width of the most tiny parts of these structures made on die that the nm is referring to.