My initial thoughs were also at what altitude they passed each other. I imagine it would be mighty expensive to run a jet under 10,000 so I'd expect they were in at least class E which, as we all know, requires a transponder.
Perhaps another point to look at might be that the closing rate of these two higher performing craft would still have, more than likely prohibited either pilot from turning away even if thy did see each other.
Perhaps our CASA friends should look at also requiring high speed aircraft to carry a transponder and not limit things to altitiude levels.
Just my 2 cents
rocket