This work demonstrates that diffuson-mediated thermal transport is dominant in Ag + argyrodite superionic conductors at temperatures relevant for thermoelectrics and ionic transport. An important consequence of this fact is that ionic conductivity can be varied by orders of magnitude without affecting the thermal conductivity. The similarities of the argyrodite structure with lithium and sodium superionic conductors means that diffusons are likely prevalent in battery materials as well. Thus, by understanding the origins of thermal and ionic transport, in particular via vibrational characteristics and phonon occupations, novel design concepts are possible to further improve functional ionic conducting materials.