Using 18:9 instead of 2:1...
...makes sense because it clearly communicates that the screen is just a smidge taller (in portrait orientation, obviously) than the 16:9 screens that most users will be using for reference.
That said, with a variety of aspect ratios it is the expression of screen *size* by the diagonal measurement that is less helpful - being told the width of the screen and phone gives a clearer idea of the phone's size (with respect to thumb reach and pocket-friendliness.)