They dont use D2 as a reference you do realise D2 just copied previous games, D2 is just an evolution of D1 and used gameplay features from previous games. Diablo didnt create the genre on loot based RPG games.
Pinnacle means being the most successful, the game on the highest peak, Diablo 2 is not that game no matter how you try and spin things, every other aRPG available currently beats diablo 2s ass every day of the week in just being a better game.
Currently as it stands, either PoE or lost ark are probably the best games of a similar style to diablo, D2 is for a 100% fact not the pinnacle of anything, the meaning of pinnicle itself has no place in games, every single game in existance can be improved upon, no game will ever be the best forever.
I have played the game multiple times throughout and the gameplay is terribly average, i have spent hundreds of hours in PoE and over a thousand of hours in D3, and D2 gameplay is just plain bad compared to a modern game.
You will never win this argument, D2 is just not the best game of its genre, and only the die hard D2 cultists would even think it was remotely even close to being the best.