I can explain this pretty concisely. While everything that you said is true, what OP means is specifically meta picks. And meta picks do not matter at all until high diamond for a good reason.
I peaked at Plat 2 playing a variety of champions, but mostly Nasus one tricked my way up there. And let me tell you, 99% of everyone that I've ever played with or against is at best OK at the game.
I've smurfed a lot. I can carry in silver with ease. And I mean, like, 0 stress, playing drunk, carry the shit out of players that belong in bronze. I can do this playing literally any champion with a full damage build. And I can 100% do this with playing every tank with a full tank build. I can do this up until high gold at which point I hit a wall because I'm actually not very good at the game. But I digress.
The reason why the meta doesn't matter until high diamond is because every player up until that point makes a lot of mistakes. I mean like, all the fucking time with your movements you are making mistakes. The reason I can carry in silver playing full AD Thresh or full AP Blitzcrank is because silver players don't know how to path or play around cooldowns.
I played Nasus in Plat 2 and against a player who can recognize pathing mistakes, a tiny over step for a CS loses the whole laning phase, and can lose the whole game. I'm not joking about a single movement as well. Because when a good player gets an advantage, they can destroy you.
Up until high diamond, everyone makes a lot of mistakes. So a player that makes less mistakes will gain the advantage. At a point where both players play flawlessly, the stronger champion will win.
Also, some champions are straight up not viable. For example the Nasus build that I played, which was the Q max, is incredibly easy to abuse in the laning phase and can get shut down to the point the game is basically 5v4 and good players absolutely will win games off of that. The E max is, for all intents and purposes, a completely different champion and helped get Quas 2 accounts to top 10 challenger.