So, does Larian have the right as DM to make Height an advantage? Yes. The rules say the DM has the right. Do advantages stack? No. 1 extra die.
Should height provide advantage? In my opinion, no. Why? Because it creates too much imbalance. That is the bottom line. If in every battle every character is fighting for the high ground in order to win, that takes away from this: "You usually gain advantage or disadvantage through the use of special abilities, actions, or spells. Inspiration can also give a character advantage. " These are the standard ways you gain advantage, not height. Height advantage, therefore, makes all the standard ways of gaining advantage almost obsolete.