Why couldn’t it be native (whatever that even means)? At the end of the day, it does have a canvas you are free to draw anything on, so there is nothing you can’t imitate.
Native would be Win32 calls to draw widgets. There's a built-in Swing look-and-feel that uses this API to render widgets in Swing. Yes, you could write your own code to replicate how things look, but it would be fragile and difficult to maintain. With Swing, it's built in and already maintained.