Neither Dynamo nor BigTable are document stores.

And there is no reason why Postgres couldn't handle this kind of stuff. Sure it doesn't come with the needed scaling tools built in but it's doable. I'm running billions of rows through a Postgres analytics setup.

A purpose built system can be more efficient but it also doesn't need to mean NoSQL. You can see some of the serious-scale datastores adopting SQL-like query interfaces. For example Hive for Hadoop or PlyQL for Druid.

NoSQL is a loose term for a mish-mash of technologies.