Definitely not intending to take credit away from any other teams. There are so many good Postgres extensions and forks in this ecosystem.
What we've done is add a Columnar storage feature into the Citus open source extension to Postgres, as part of Citus 10.
One way to think of it: Citus 10 now gives Postgres users (those who are also using the Citus extension) a columnar compression option, for use on a single node and/or a distributed Citus cluster.
Thanks. Regardless of who did what, columnar stores are non-trivial engineering and anyone who produces a production-ready one is worthy of admiration.
I was the PM for the software side of Greenplum DB for a while. I also don't know of an earlier example of a separate column oriented storage option on top of a Postgres base. Vertica and ParAccel (which became Redshift) both also started from a Postgres base (though Vertica claims otherwise in marketing material) but were a little later. In any case, "Column oriented storage for Postgres" is almost 20 years old. This also isn't the first open source implementation.
Depending on how one defines that claim, Greenplum may have been first.
Disclosure: I work for VMware, which sponsors Greenplum development.