The first time that deep black clay is excavated, it always stinks bad! - If it is broken up and allowed to dry out, the stench goes away. The stuff is loaded with rotting organic material.
Horner’s comments referred to the fossils themselves, not the matrix. That said, hydrogen sulfide odor is a redoximorphic feature associated with organic processes - fully developed clays are by definition mineral, not organic, are rarely black (never, in my experience), and basically odorless.