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.
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.
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.