-_- you don't know what you are talking about here
for car reflections Forza, like many racing games, uses a dynamic reflection map that displays a lower quality render of the surrounding environments in real time.
it does not use screen space information for these reflections whatsoever.
in the part I timestamped you can literally see the plane on the car's reflections before it enters the screen.
in Horizon 4 dynamic objects would not be reflected in these (from what I know, maybe there were exceptions I didn't notice).
for example, as soon as you hit a small tree that can be knocked down, it would disappear from the dynamic reflection map as soon as the static tree model is switched out for the physics based destroyed model.
same with walls and bushes.
This is the first time I ever noticed a dynamic/moving object in the dynamic reflection map of a car in a Forza game... but maybe they always had that in show races like these and I simply didn't notice