This is an often misquoted fact. The study that compared coal and nuclear was only studying air pollutants, and obviously the steam stacks from nuclear reactors don’t have as much radionuclide pollutants as coal. However, the study did not look at other sources like wastewater - which is where most radionuclide pollution from nuclear reactors comes from (along with other sources like spent fuel, casings, and moderator rods).