I am not good at this game and I am way higher placed at the rating than him. Thus I have to think he is bad as well.
There's nothing that says you have to discredit other players achievements in order to acknowledge that you aren't as good as you'd like to be.
Thanks to playing so much of this game, I know there's a lot I don't know, therefore it feels like I'm bad at the game (and I say it a lot in game as a self-deprecating joke). However, I also know that I have the knowledge to know that there's a lot I don't know, which makes me a lot better than I would otherwise give myself credit for. https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect
Mid diamond (my rank) or platinum (OP's rank) isn't impressive to most people on this subreddit, but that's because most people on this subreddit have inflated expectations (and are more likely to be higher elo than the average population of League players, because they're more invested in the game).
However, if I said to anyone else "I am rank ~20000 on a ladder with ~2.5million people", that would be incredibly impressive to them.. because it is impressive.
I really dislike how players, particularly those who are slightly above my own elo (let's say master tier) consistently put down others and say that hitting diamond or platinum isn't impressive because "you know nothing about the game at those ranks" when you clearly do and are clearly better than average. It reeks of elitism.