Old and new Campy 10 speed compatibility

isn't it to do with the increments of cable pull? Hubs are just a width? 9 and ten use the same over width, just the shift increments are different

Yes. The hub splining for 9-10-11 is the same, so you can fit a 9 speed, 10 speed or 11 speed cassette on the same hub. The rear mech compatibility is absolutely to do with cable pull and the width between sprockets is different for each of 9, 10 and 11 speed cassettes.