I see that you are considering only the light, but there is more than that. Time dilatation around gravitational fields is real (we have it on Earth too). Given

I see that you are considering only the light, but there is more than that. Time dilatation around gravitational fields is real (we have it on Earth too). Given the extreme setting in this regard around a black hole, it may be possible that, even if the falling object has not yet arrived at the event horizon, there to be a significant and increasing time frame difference between that of a distant observer and that of the object moving closer to gravitational center, which lets the observer see a progressively slower fall. I find this to be more likely the reasoning behind the novel's depiction.