huh? why would suburbs magically be exempt from that idea?

Yes, places grow, this is why it's important to apply good urban planning and use as much high density housing as possible, otherwise you get the miserable car-dependent sprawl we see in america and much of the rest of the world.

By centering around transit stops you get rid of the need for all the parking and roads that takes a ton of space (which lets urban areas be smaller while containing the same amount of living space), and by having many small towns with high density centers spread out like this you maximize how many people can live close to the countryside.