How I interpret things regarding the platforms is that yes, there are no platforms at Willesden Junction that are in use on the original London & Birmingham Railway tracks.
However, the present day DC line was constructed by the later London & North Western Railway, of which the L & BR along with the Grand Junction, Liverpool & Manchester, North Union, Lancaster & Carlisle, etc.
Furthermore, it is not possible to travel on a direct Intercity service to Willesden Junction, due to there being no platforms on the original London & Birmingham Railway metals. So, in other words, I would have the view that it is not a doubleback at all.