I think they explain very well what their assumptions are. Donald in Mathmagic Land discusses spin, so I don't think they missed it.
No spin involved. From the article:
They typically assume that their billiard ball is an infinitely small, dimensionless point and that it bounces off the walls with perfect symmetry, departing at the same angle as it arrives, as seen below.
Without friction, the ball travels indefinitely unless it reaches a corner, which stops the ball like a pocket. The reason billiards is so difficult to analyze mathematically is that two nearly identical shots landing on either side of a corner can have wildly diverging trajectories.
Pool balls don't work like that. Not even close. The math "billiards" has almost nothing to do with real billiards. The people who study this stuff are happy with 99-rail shots or even 1000000-rail shots.