I'm kind of silly even responding to this, but, consider the example of chess: intelligence actually has very little to do with predicting how good someone will be at chess outside of how fast they learn to play and the outcomes of their earliest games. Experience matters more than intelligence, to the point where someone can be a grandmaster with completely average intelligence.
This is because in games, it's more a question of -knowing- what to do in what situations than figuring out what to do, especially when the game is complex and there's simply not enough time to calculate out everything involved. In League, especially, there are a very large amount of situations that you can be in and need to have knowledge of. Having a high IQ won't let you know whether or not you have enough damage to all-in Sion as level 7 Irelia, for example. Only experience really tells you that.
EDIT: This also means that someone with adequate knowledge of the game can probably first time a champ with no issue, since that's being brought up. If they're already familiar with the champ's damage, kit, teamfight, etc. from experience, they can likely replicate that to some degree, especially if the champion is mechanically simple. That doesn't mean a challenger player will first time a champ at challenger level, but plat/low diamond level is hardly unreasonable.