A Controversial Theory
People in low elo are not neccesarily bad at their best champs, they're just bad at the game.
Let me explain.
Relatively difficult champions that aren't the absolute most meta picks and/or are only played by onetricks have relatively little change in winrate between Silver and Plat+, according to champion.gg, despite the playrate remaining about the same.
Orianna, Gangplank, Draven, Zed, Bard, Thresh, Yasuo, and more are all good examples of this. When looking these up, the only time I was wrong was for Taliyah and Aurelion Sol - both have noticeably high winrates (and playrates) in higher divisions.
Lower elos drag down winrates when actually difficult champions become meta, and people in low elo switch over to them and feed. Azir and Nidalee are good examples of this.
I can also remember a time when Orianna was considered a very difficult champion, one that low elo players should avoid. But if you think about it, whats gonna stop a Silver mid main from getting pretty good at Ori after a few games? The mechanical challenge of landing ults and moving the ball around while maintaining good positioning is something that you get from being comfertable with videogaming overall.
I think that, as League has gotten more popular, the average mechancal skill of players in every division has gone up. Keep in mind that 4 seasons ago, Lee and Ezreal were considered hard champions. When Syndra was released, she was considered hard. TF and Alistar also used to be hard champs.
TL;DR over the years, the mechanical skill of all League players has improved to the point where classically difficult champs are just as good in low elo as they are in high