Simple explanation of what OP means

If a champion's winrate is too much higher than 50%, they're overpowered and need to be tuned down.

If the introduction of an item brought multiple champions' winrates far above 50%, it's overpowered and needs to be tuned down.

The numbers prove that the change brought about an unfair advantage in playing that champion or buying that item. What it does not mean is that suddenly every single player in the game forgot how to counter-pick or counter-play against that champion. That's completely unreasonable.