A safety driver is a driver that needs to stay focused on the vehicle's movements 100% of the time, to intervene if the autonomous system fails and starts to become unsafe. This is not what Waymo uses. The autonomous system is trusted with safety, the remote agents are there to resolve cases where the autonomous system cannot proceed (or, more precisely, is not allowed to proceed out of an abundance of caution) without some extra guidance. Whether Waymo's business model makes sense primarily depends on how often these agents need to intervene (unlike safety drivers where you need one safety driver per vehicle and you're just running an expensive taxi).