Plaster and gypsum board (drywall) can be used together. The author's own house uses plastered gypboard, and from what I understand plastered gypboard is still extremely common in Europe and around the world.
We had rehab work done when we moved to a Boston area house and they skim coated everything with plaster as standard of practice. That is definitely not the standard anywhere else I have lived.
Another regional difference is they put the powder room light switch next to the door on the outside, why? Has anyone seen this in any other area?
It's a Massachusetts standard, explained to me as an early safety procedure before GFCI was invented -- and probably dating back to early exposed wiring. Separation of water and voltage.