Here's how they do it
I asked in another forum (populated by semi guys) and got the answer. They deposit alternating thin films of two materials, position some hole sites using a single exposure, then etch boreholes down through all the layers, then fill the holes with metal. That gives you a stack of gate all around transistors in series. Pretty cool, and nothing like either stacking dies or processing 100 metal layers.
The reason they're using somewhat older processes than this is because the holes can't be perfectly cylindrical, but are conical to some extent, so if they start too small at the top they'll dwindle down to nothing before reaching the bottom.