I realize it's the tagline, but it's kind of misleading to call Urbit an OS when it's a program that needs to run on Mac or Linux. WeChat is more fully featured, but we don't call WeChat an OS...
The term "operating system" is far more precise than "fully featured"; in other words, the accretion of a certain number of features does not constitute "operating system" at some point.
From Wikipedia: "An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs."
From Wikipedia: "An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs."
It does that, which you can read about here: https://urbit.org/docs/tutorials/arvo/arvo/
Whether it's hosted on top of Unix at the moment is beside the point.