Especially with such a small number of rigid bodies to worry about. The literal only thing that has to collide with anything else is the pinball itself, unless we're talking about a pretty weird pinball machine.
Of course, but that rigid pinball has speeds in three directions, rotations along many axes, friction might change on different surfaces in the simulated machine, the ball can jump, bounce and slip depending on how it's rotating and if there is more than one ball in play then something like a bumper can "fully" hit one ball but merely graze another one. Pinball machines are a prime example of what the article talks about: a basic simulation is indeed quite simple. A good simulation rapidly spirals down into the most obscure details.
