Because most people refer to any aftermarket suspension as coilovers. As stock, the 350/370 run separate dampers and springs at the rear which 99% of aftermarket solutions copy, hence they're not true coilovers.
This is why I'm picky over stuff like this: It's not (just!) being pedantic, it's ultimately misleading and leads to confusion. It's like saying that when you have a puncture by the side of the road you get the jack out and change the tyre, when actually what you do is change the wheel. I don't care if most people know what you mean, it's still wrong!