[kvdveer]

While that could be an acceptable simplification on the aforementioned Amiga, it breaks down when multiple balls are in play, if the friction between the top and bottom plate differ (introducing spin) or when the ball doesn't hit the bumper exactly straight on the y axis.

Generally speaking, there are many ways you can simplify the game by sacrificing precison. This is exactly the kind of detail the original article is about. Ones simplified model of the world is always incomplete, and there are always surprising ways in which it will fall short.