Correct, the lunar soil is itself radioactive because it's subjected to neutron radiation. What's more, the radioactivity of lunar soil increases with depth; the Apollo 17 Lunar Neutron Probe Experiment found that the radioactivity peaks at about 1 meter under the surface. I believe this is because the lunar soil moderates the incoming high energy neutrons (thermal neutrons are more likely to be absorbed.) This effect means that careful planning would be required when shielding a Moon colony. If you just haphazardly bulldoze lunar soil onto your habitat, you could easily make your problem worse.
