That demo repo https://github.com/MangoDB-io/example is forked from https://github.com/mariadb-corporation/dev-example-nosql-lis... which "allows a MariaDB server or cluster to be used as the backend of an application using a MongoDB client library".
So in other words the MangoDB project tries to port a MariaDB NoSQL example app to PostgreSQL.
EDIT: seems I mis-interpreted the Github repo. The example is indeed a fork of the MariaDB project but the underlying MangoDB is not.
When I hear of "MangoDB", I remember this: https://github.com/dcramer/mangodb
Funny how there's an actual project with the same name.