The hijackers would probably want to make it public. It's an insurance for their safety right now. A government might simply bomb the parked aircraft and blame it on a crash or the hijackers.
There are ways to cover that - not all the hijackers will be with the plane. All they've got to do is keep some proof of the negotiation for the survivors to publish and that would be easily disprovable.
Besides, if they knew where the plane is to bomb it this would be a hostage situation right now. They might be in a country which wouldn't welcome the incursion but I can't see the Chinese being overly bothered about that given the number of their citizens on-board.
Because publicly people don't negotiate with terrorists.
Now we all know that that isn't true so it may be they felt that keeping it quiet (for now at least) would give them a better chance of getting what they want.
