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."

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.