And, from the docs:
Greenplum Database is based on PostgreSQL open-source technology. It is essentially several PostgreSQL database instances acting together as one cohesive database management system (DBMS). It is based on PostgreSQL 8.2.15, and in most cases is very similar to PostgreSQL with regard to SQL support, features, configuration options, and end-user functionality. Database users interact with Greenplum Database as they would a regular PostgreSQL DBMS.
That means that essentially we're talking about a fork that occurred so long ago, that the version from which they forked on is no longer even supported upstream. It was released on 2009-12-14. That's over seven years ago. PostgreSQL only supports 9.2+. At the very least, this should not be tagged PostgreSQL. Ideally, I would like it to not be tagged postgresql-8.2 either but because no one uses that tag, I agree the issue is somewhat moot.
This was inspired from this question which is tagged, and titled PostgreSQL however it has nothing to do with PostgreSQL. It's a Greenplum question, and it has that tag too.
The tag wiki for postgresql says the tag is supposed to be for all versions of Postgres. Simply put, Greenplum is a fork. It was never a version of PostgreSQL.