Didn't they have those "old" Imperial access codes, so the shield would be lowered? The moon must be shielded too, or else they could just slip in on the far side away from any traffic around the DS.
Other Imps like Tarkin seem to suffer from overconfidence. In the conference room scene from SW, there is debate how strong the Rebel forces really are just before Tarkin and Vader walk in.
Ozzel might have been foolish enough to think a big ass Super Stardestroyer, (which could have been a new vessel) popping up on the Rebel's doorstep would be enough to terrify them into not putting up much of a fight.