Well no, the SAARU is not a backup for the navigation function of the ADIRU - it only handles attitude and air data, hence the name. Without the ADIRU, GPS supplies position to the FMC, so you can at least still fly your way home using HDG SEL. I suppose its just a general precaution - you already lost a backup for your attitude and air data, you shouldn't also lose groundspeed, track and position while you're attempting to fly yourself home on the ISFD.