They are. Even official Australian Government websites say they are.
The GDR obviously has many spurs which are a natural part of any range. Every spur has a water course that run parallel to it (that's how the spur was made) and as you said earlier, the the BM's constitute a spur that runs off the GDR.
Even Tasmania is considered to be part of the GDR as the so called southern spur. On that basis I think we can safely say that the BM's are indeed a part of the GDR. Never heard or read anything to the contrary - until this thread.

