If you're running a "real" cluster (i.e., where you actually need multiple machines instead of just one big one running postgresql), then yes, the recommendations there are realistic -- not because cassandra is inefficient but because you will ipso facto be dealing with a large dataset.

Cassandra does scale down (I know at least one site that went live running Cassandra on a 256MB VM) but that's mostly of academic interest to the audience looking for a scalable database. Hence the recommendations on that page.