One must thank them for open sourcing this, and cannot blame them for using a different license, but using a different license makes me think calling this "unfork" is bending the truth a little bit.
The "unfork" part is primarily about not forking the postgres codebase anymore, as done before citus 5.0 (i.e. we modified parts of postgres, to make it citus). Citus now entirely works as an extension to postgres, using the extension facilities postgres provides.
Perhaps I'm missing something, but this is just an extension that works with standard postgres, there are no code changes in postgres itself, so it doesn't look like it ever was a fork.
Yes, that's what you're seeing right now, but in the past Citus (used to be "CitusDB") was a superset of the entire PostgreSQL codebase. During the lead-up to the open source release, we removed the use of any static methods or internal machinery and rewrote the installation process to use the PostgreSQL CREATE EXTENSION command. Additionally, we moved all of pg_shard's DML functionality into Citus to unify the product line.
So ultimately CitusDB was a fork but is now entirely an extension.