By not enforcing the fugitive slave act those ‘passive’ states were engaging in nullification
Sort of.
When I think of nullification I think of an official act, such as South Carolina’s 1832 Ordinance of Nullification. They stood up and said very unambiguously that they weren’t going to go along with the federal Tariffs of 1828 and 1832. On the face of it the nullification act wasn’t sustained but the net effect was prompting congress to modify the tariffs.