It's not part of any standard, so they'd need to pick what to implement. They docs say they're modeling it on postgres's, which is a perfectly reasonable extension.
True, though SQL isn't much of a standard anyway. I literally don't know of a single SQL database that implements the SQL standard to spec with no fiddling. Or more precisely, the vast majority of SQL databases out of the box behave differently, so anything more complex than a basically SELECT + JOIN generally isn't very cross-compatible.
