Why? Seriously, JSON is lame, it's an idiotic trend, a fashion from the UI that's infected the back end for no gain whatsoever for anyone not using JavaScript. XML is better and we've just watch JSON slowly reinvent everything XML already had.
My guess is the grandparent is (mis-)remembering the kerfuffle[0] around mongo shipping a copy of PG as a "BI Connector". But yeah, the timeline is off, that was in 2015.
Actually, no. I'm remembering asking myself how Mongo worked back when it launched, and reading somewhere that it was a heavily modded Postgres under the hood (like Amazon Redshift, which makes no secret about it, since).
I might be misremembering, though. And there's a possibility that what I read then was inaccurate.
I might admittedly be remembering this wrong, but I seem to recollect that Postgres had a couple of (unofficial) contrib modules to support JSON back in 2008 or so, and that (unfortunately buried in Postgres vs Mongo articles) the initial release of Mongo was actually a fork of Postgres for all practical intents.
The relevant data structures and indexing mechanisms (hstore, tsvector, gin, gist) were all available -- if only experimentally -- before Mongo launched.