The narrator who mentioned the Mule is Gaal Dornick I think. My suspicion is that the Second Foundation is the missing part of the equations and that she is tasked with ensuring its creation. The Mule appears at about 300 FE in Foundation and Empire so unless they accelerate the chronology, she somehow contrives to be around for at least that long and her tone makes me suspect she helped engineer the neutralisation of his powers. There are a few SF technologies that would make it possible for her still to be around but none are associated with Asimov's original trilogy as far as I recall.