Indeed, not only can one create multiple databases on one RDS instance, but it's also OK start an RDS instance with no user database at all, and create it later.
We're heavily using multiple databases on a single Postgres RDS. Yes, occasionally it's annoying not having a true superuser, but now they've improved it so you can still have 'fake superuser' reserved connections it's ok.
The limitations are presumably so that they have control over settings for things like replication and their features that you can't mess with.