I don't believe that is true. 900 - 1000 MHz are line of sight frequencies and won't allow a plane in one valley to detect another in another valley. I appreciate that Australia is rather flat and maybe there aren't valleys, but in the US there have been collisions between fire-fighting aircraft that should have been avoidable.
What I specifically said was that planes with no ADS-B Out would have ADS-B out synthesized for them by ground stations that are using primary radar to track them. There isn't something that makes this sequence impossible. If there is no primary radar, a separate sequence, that is a different problem.
As to the last bit, yes. It is very helpful to have ADS-B Out on all manned aircraft. No disagreement there.