Might not be what they meant, but under normal operations a coal plant can release significantly more radiation into the environment than a nuclear plant, via isotopes in the ash [1][3]. It’s still less than normal medical sources but it is much higher than a normal nuclear plant where radiological releases of any kind are actively watched for. (The source of the coal will determine the isotopes involved and the initial amount. [2])
This article [3] from Oak Ridge National Laboratory REVIEW (1993 I believe, so a bit dated) states "The population effective dose equivalent from coal plants is 100 times that from nuclear plants"