This is a Google-internal only GA. JJ is available externally just fine. Google is mainly a linux-dev shop, with all other platforms being second-class citizens.
The main exceptions to this are devs who work on iOS or macOS software, who will sometimes do local builds on their physical machine. They would benefit from jj support, but there are more hoops to jump through, and the jj port will most likely be less about running on macOS and more about jj supporting the weird ways in which source is accessed.
... but Google solves the "A Linux fleet requires investment to maintain" problem by investing. They maintain their own in-house distro.
Not really, it is just a well known outside distro plus internal CI servers to make sure that newly updated packages don't break things. Also some internal tools, of course.
Well FreeBSD exists, just look at Netflix