OP is correct. Also shooting from above adds an additional dimension to the movement. Shooting someone on the same height as you, you don't care if the run in a 45° angle towards you - its just left or right. From above it does matter. The biggest advange from shooting from the highground is the additional overview, cover helps less and the guy below needs more energy to reach you physically (also with his/her projectiles).
Just test it in any 1st person shooter
Or read what Total War has as advantages/disadvantages... or just go with the 5e rules. In short BG3 solution is oversimplified and unbalanced.
