None of the plastic 120 spools I have here, which are mostly Kodak and Ilford, have a hole going all the way through axially. When I put an depth gauge or the point of a small screwdriver into the drive slots at the end, it is clear that the part that obstructs the central axis is fairly thin. That is, you can think of there being a thin "cap" that closes off the end of the drive slots, between the drive slots and the central area with a slit for the paper leader.
The metal pins he's using to drive the 120 spool seem like a good idea. On plastic 120-to-35mm adapters, the plastic tips that fit into a 35mm cartridge are pretty small and could break off (it hasn't happened to me inside a camera, but I have an adapter that was dropped and one of the tips broke off).