Unfortunately, it is not. You just aren't stressing your bones enough (muscles, yes). I am in a similar situation as you; living in New England, in a place where there is little flat riding. Those hills are tough, but they are not building bone density. In fact, there's research that shows many cyclists actually lose bone density because of all of the sweating, with no impact.
Not good for someone like me, who already has osteopenia. Weights, Pilates, hiking, maybe yoga will do it.