Money laundering 101: breaking the transactions up into smaller and smaller amounts making them more and more difficult to track
I'm curious if Zhao discussed with some of the mixers' owners to block those funds. That would be an interesting move. Then the hacker may end up with a pile of Bitcoins he cannot ever use, unless he'll start sending to unsuspecting fools really small chunks. Or simply with the money seized by the mixer and returned to CZ.
He doesn't have that kind of power , mixers are out to make money and a lot of their customers are involved in shady activity. There are many different mixers that the hackers will be using and they will obviously do it in small chunks if they are smart.
Its easy to get away with stolen coins if you have half a brain and are patient.