Most "mains" are just people who've picked up a champion, are Plat+ and say "oh, this is what's wrong" when in reality they are just saying things that are either half-true or go with what others are saying. Take the current Lulu for example, there's people who've put hundred and hundreds of games on her, yet they're drowned out by the people who've suddenly picked her up and say "well, here's your issue right there.' It's like hiring a mechanic fresh from a trade school.
Most actual mains I've encountered will mostly say they're strong due to being able to out-duel opponents (myself included). However, what sets these people apart is that they're open to seeing slight buffs or nerfs to them (ie, would say a lot of Ahri mains don't really care about the MS buff on Q. I myself don't really notice it that much and probably wouldn't miss the thing myself)
As well, they will tell you exactly how to counter them, point for point. It's not always "just cc them," but actual strategies when in lane and late in the game. Last Ahri example would be that I tell people if she misses a charm, they should immediately turn on her as it's an important cooldown as well as she doesn't deal well with fighters.
Just because you play one game doesn't make you an immediate main.