It is a quirk of Starwood's reservation system that your status at the time th reservation is made is sent to the hotel, and not your status at the time of stay. Of course this works to your advantage when your status drops. Also some hotels have their own computer system which records your status from your last stay at that hotel (even if years ago) and uses that instead of the SPG status. Very quaint system.
The Sydney 4PTs was built as a Nikko, in the japanese hotel style. This means there are a dozen different sizes of rooms (to fit the Japanese social hierarchy), and half the rooms are pretty awful. The smallest room is 1/4 the size of the largest room, a size range that is rare at other Starwood hotels. Plats normally benefit from this, getting the nice and large rooms, while those with no status find this hotel to be far less nice.
If it is any consolation, all of the hotels near SYD airport are pretty awful (and very over-priced), so you probably were better off.