That "Review of Urbit" is the best post I've read on it, thanks for sharing.
people here are describing Urbit like they're discussing the meaning of the monolith in 2001, or the true meaning of Christmas. I like your take because you're describing what the code is, rather than the objectives or use-cases. it feels like TempleOS to me because (to me) the code is the point. it's an exercise towards creating a specific architecture as an end.
In summary: Urbit is a functional/deterministic VM. That's mostly it. It runs a bytecode called Nock. The only language that really compiles to Nock is a weird language called Hoon. Because the VM is functional and deterministic, it is (in theory) also portable – you pick up the (ever-evolving) VM image and move it somewhere else, and it should act the same. Since the VM may move around there's also an identity/networking layer so you can talk to the VM wherever it is.
Right now Urbit is a program called u3 (runs on Linux or whatever) that runs Nock programs. OS 1 is such a program that does a couple things. It's unclear to me if they've really built an OS or just a library of routines that can be used to build a single multipurpose application.
The most direct article I found was "Review of Urbit": https://medium.com/@noahruderman/review-of-urbit-e7cc4c35f14...
Urbit's own "Common Objections to Urbit" was helpful, in a methinks-he-doth-protest-too-much sort of way: https://urbit.org/blog/common-objections-to-urbit/
And that article pointed me to "Houyhnhnms vs Martians" which was helpful though also as weird as Urbit: http://ngnghm.github.io/blog/2016/06/11/chapter-10-houyhnhnm...
While some people say Urbit is feudalistic, I tend to agree more with those that call it simply obscurantist. More occult than fascist.