Thing that only just now occurred to me:
Why would anyone on the Millennium Falcon be surprised that the Death Star wasn’t a moon? Moons orbit planets. There are no planets around. Alderaan never even had a moon to begin with.
If I remember rightly there was a line in the novelisation about it. When Luke says “heading for that small moon” Han says (something like) “strange, I wasn’t aware Alderaan had any moons”…