That's a pretty broad statement, and one that I think misses the point of the ND and ND Grad filters completely. We're actually trying to let less light in with those filters and doing it on purpose for the sake of the shot we're composing.

While it is true that without light there is no image to capture (and IR folks might even dispute that) it is completely possible and often necessary to artifically limit the light to capture a shot. Film and sensors don't have nearly the range as the human eye, so they occasionally need some extra help to capture the scene.