They did do that, yes, but reSTIR is not pathtracing. ReSTIR is a lighting integration method that can choke down an enormous number of lights, far more than lumen can currently support, which means the lighting approximation is more accurate.
All reSTIR is, is an algorithm that tells the renderer how to choose which lights will contribute to a given pixel or probe. It works in path-tracing, lumen, and basically any problem that boils down to stochastic selection.
That said, ReSTIR is also the algorithm that makes Nvidia’s real-time path-tracing possible, because without it, it would take way too long to get clean data to light a scene.