So *IT IS* using an Amiga Native UI, and not its own?
Depends. Please define "native"... :-) In principle it uses the Amiga native port of XUL - which was btw one of the reasons for makiing this port, 'cos with a native running XUL enviroment it is possible to port other stuff more easy. In itself it utilizes of course native API like graphics.library, intuition.library, BOOPSI, maybe cairo etc. - so it doesn't depend on a specific GUI *toolkit* like MUI or Reaction. Just plain, unwrapped OS functions (this is all AFAIK and might be partly wrong...) which translate the GUI descriptions from XUL into a user interface.