We investigate the effect of interfacial exchange coupling on broadband ferromagnetic resonance (FMR) response of thin film IrMn${}_3$/NiFe bilayers, focusing on the evolution of effective magnetization ($M_{eff}$) and rotatable anisotropy field ($H_{ra}$) as a function of IrMn${}_3$ layer thickness ($t_{AF}$) from 0 to 4.5 nm encompassing the paramagnetic to antiferromagnetic (AF) phase transition of IrMn${}_3$. Our results reveal a $$4% enhancement in $M_{eff}$ upon formation of AF ordering in IrMn${}_3$, without a corresponding change in the volumetric magnetization of NiFe. Theoretical modeling indicates that this effect arises from the hybridization between the standard FMR mode of NiFe and the fundamental mode of the IrMn${}_3$ layer, which is activated by interfacial exchange coupling. Because the fundamental mode frequency of IrMn${}_3$ lies far above the GHz FMR range, this hybrid mode penetrates the IrMn${}_3$ layer as an evanescent excitation, making $M_{eff}$ predominantly sensitive to the interfacial exchange energy rather than to the bulk AF parameters. A control calculation in which the AF moments are held fixed produces only a resonance-field shift and no enhancement of $M_{eff}$, demonstrating within the model that the observed increase requires a dynamical AF response. Our results establish $M_{eff}$ as a valuable tool for probing AF spin excitations, given that it is a more phenomenon-specific indicator in comparison to other FMR parameters that produce homogeneous resonance field shifts.