When penetrating a concrete fire-resistive wall or floor assembly with rmc, it is not acceptable to use mortar to fill the annular space?

This statement is not true

There are two main categories of fire-resistance-rated assembly penetration: Membrane penetration and through-penetrations. Items through penetration pass completely through the fire-resistance-rated assembly. As compared to many other codes provisions, requirements for through-penetrations have their exceptions.

For example, IBC allows penetrating pipes of steel in fire-resistance-rated walls to have the annular space which is the gap that exist between the penetrating item and the floor or the surrounding wall filled with concrete, mortar in masonry or grout.