False - there is more diversity mid/top/jg. You don't have the same 5 champs being played every game. And even if you did - it doesn't mean the same thing.
Allow me to make up some hypothetical example to illustrate the point. Suppose for a week, somehow the winning team always picks malphite top. Now this malphite is the same malphite we see in solo queue today - no changes to build or buffs/nerfs to the champ. This would leave malphite with an insane climb in winrate. Thus winrate would say malphite is broken, although no changes have been made to him. From today's games, we know malphite is not broken nor is he the top tier toplaner. Thus while malphite was obviously not the sole or main determining factor in most of those games, he appears to be. This illustrates - albeit fictionally - how winrate and being the main factor in the outcome of a game/being op are not synonymous - they are correlated, but not cause and effect.
Secondly - adc's are highly influential and powerful. Few would argue other roles are more influential mid/late game - especially in teamfights. However, in some games, a team might have a much better frontline PLUS one or two good adc's and a second damage source that can pop an enemy squishy in one combo or with only minor followup damage. This is much more likely a scenario than the hypothetical example above. Now, the other side picked carries and/or early game champs in ever role: jayce or renekton top, elise or ezreal jungle, lucian mid, and traditional adc+enchanter support bot lane. Because the second team has no frontline, they lose - despite adc's being very strong. In such a game, we have 2-3 adc's losing and deflating their champ's winrate at high elo depsite adc's being so strong. Because adc's are played in so many roles and likely multiple roles over the course of different games (lucian top, lucian mid, corki adc, corki mid; you get the idea) - they are more likely to experience several losses over the course of a week than a champ (like warwick or poppy) who is played in basically one role - even though it's technically a 50/50 that said champ end up on the winning side. There is more guarantee that an adc end up on the losing side of at least some games because they are played in so many more roles and more frequently.
I know I'm not doing the best job of explaining it, but it's the best job I can do.
