I think it's just because the dive try was so early in the game that if it hadn't been awarded the whole match would have been different - who's to know what the result would have been. Whereas the Tizzano clear out was in the final minute, so there's a clearer sense that if you cancel that try the result is different (albeit the Wallabies would have still had to kick for touch and navigate a lineout before time would have been up).
I was interested to see if there's ever been a result overturned in professional sport due to a refereeing error and found this interesting case from the NBA. In 2008 the final 51.9 seconds of a game between the Miami Heat and Atlanta Hawks was replayed 3 months after the original game - right before the start of their next scheduled game that season. Shaquille O'Neal had been fouled out of the original game at that time, but it was subsequently discovered to be an error (it was recorded as his 6th foul when it was only his 5th). The league upheld the protest and so they replayed the final minute, though the score ended the same.
But hey, that's a clear precedent in global sport. If the clean out is deemed illegal I'm sure the Lions will agree the honourable thing to do is to replay the final minute in Sydney before we kick off game 3