You don't have to start with hay, no matter what. Hay is grass, dried grass(except for alfalfa).Are you going to try to feed all grass? or like a treat? You will have to start with hay no matter what.( untill 6 months so they can grow )
If you haven't fed it before, you might need to. It depend on the rabbit. But I think they meant that you don't need to wait until 6 monthsDont you have to transition? LIke you do with pellets?