It's an idealized model, accurate if Earth had only a single all encompassing deep ocean. Idealized models are good pedagogic tools to build corrections upon.

It's similar to depiction of projectile motions as parabola s. The trajectories of artillery shells ar not like that, but helps get started.

Well, a single all encompassing deep ocean of something in which waves could travel 1600 km/h, since that's one of the major constraints, too.