Of course you can push in the scrum, you just can't go off early and with the ridiculous scrum feeds these days that doesn't leave much time to spring this move. If used in moderation and when a team's concentration is lapsing it is worth a shot.
Souths actually tried it near the end against the Dragons last week but the ball got out fast enough just before the pack got pushed back over it.
As someone said above there was multiple penalty options available to the refs in that scrum but I think they were too surprised themselves when it happened.
I'm pretty sure that the bloke who picked up the ball was packed (and I use that term very loosely) in the 2nd or front row. How can he pick the ball up without having unbound from the scrum before the ball came out?
If teams tried this tactic once or twice a game maybe the scrum formation/binding might improve a bit as teams would need to be ready for it.
There was an aerial shot of a scrum in the state of origin that showed what a joke the lack of binding has become from the lock right through to the front row.
Anyway, scrums are a lost cause so on with the show..................