ETOPS originally was an acronym for Extended Twin Operations. While it has been changed to just Extended Operations, the focus is still on twin engine aircraft. South African Airways currently operates the route with an A340-300, which has four engines. If it has an engine failure, it is not as much of an emergency as in a twin-engine aircraft, so it does not necessarily have to adhere to the same ETOPS rules for alternates.
Johannesburg to Perth is certainly a fairly remote route. But at least eastbound it's less than 10 hours, and, for example, the A350 is certified for 370 minute ETOPS. This is 6 hours, and while that is at a one-engine cruise speed, it's still fairly close to covering the entire route.
Looking at the Great Circle Mapper, a 240 minute ETOPS would be enough to fly the route with only a slight diversion from being direct. This is with 300kt diversion groundspeed. From the map, it appears that Mauritius and Cocos Island could serve as alternate towards the middle.