You're right that Addamark wasn't open source and wasn't a storage option, but it was indeed built with a Postgres base.
Here's the paper (2002):
https://www.usenix.org/legacy/events/lisa2002/tech/full_pape...
There were non-Postgres SQL columnar stores as well, most notably Sybase IQ, which dates to the 1990s: https://en.m.wikipedia.org/wiki/SAP_IQ