I think he's talking more about the entire UI. Menus and things like that.
Rockbox's design philosophy pretty much makes it impossible to recreate the Apple User Interface. In my opinion that is a good thing. The current design of the system also makes custom menus (Your "user interface") also impossible, which is debatable but probably for the best.
My suggestion? Learn the Rockbox UI. It's different, sure, but you'd be surprised how nice it is when you get used to it.