What I think Rob is referring to–and which I agree–is that one can imagine a musical result without hearing it first. Many composers do not even need a keyboard to imagine and create their music, and music came into being for thousands of years with just the voice or a keyboard to use.
It is nice to have modern electronic means to simulate and create or confirm new sounds, but I would say that most people imagine what they want and then (if necessary) search for a means to reproduce it. They do not need the electronic (or even acoustic) crutch as a prerequisite to imagining it.