That does make more sense when put that way, I agree. In other words, it refers to situations like Monstrous Creatures dipping their toe into area terrain for a 5+ cover save.
The primary scenario the pro +2 cover save argument describes are models in Ruins with bases, in which case the book tells us to treat the base as area terrain. It doesn't make sense however because Going to Ground in an identical ruin with a base would offer more protection than a building which has none. Furthermore, the ground floor is then offering more protection than the next level up, despite it being made from the same materials and composition. This promotes modelling for advantage and is obviously not in the spirit of the game but only for those seeking to power game and rules lawyer text to win at all costs.