TIEW - I too am waiting to find out if the CA/GRO heard the transmission from the Jab, however, I do not agree with your conclusion.
1. If the CA/GRO knows the tracks of both aircraft then he/she knows they will intersect, regardless of the directed broadcast from the Jab, so they are mutual traffic. That is why the aerodromes share a CTAF.
2. 500 feet vertical in cruise is a far different risk to 600 feet with both aircraft on descent. The second is pure chance.
The incident has also proven that for Australia to rely on TCAS to avoid providing ATC is going to end in tears.....