Advantage is not the only bonuses highground could give (and it shouldn't give advantage).
Highground has to matter even if it definitely shouldn't be a god mode. Delete the whole concept of verticality and positionning for the sake of "D&D 5e" is a bad idea.
And the point is, like he stated, the difficulty would be "D&D 5E mode" which would entail, implementing 5e rules, height giving Advantage not being one of them.
In his scenario, f you still want to play the game with Larianisms, play on story mode.
