mechanics can only get you so far. There is such a thing as skill ceilings. In a 5v5 team ranked game, mechanics is actually important.
But game knowledge is by far the best way to win or lose. Its everything about abiltiies, items, champs powerspike, champ dmg output--when to tower dive, when to baron, when to split push, who to target, when to peel, how to cs undertower (which is game knowledge) champ select for comp or counter comp (for the enemy) which summoners, which runes, which masteries, trading patterns. abilities maxing knowledge, what to do when ahead? when behind? when to roam, how to max your farm at all time, obj control, warding spots,
Match ups, etc..etc..etc...
Mechanics is just about how you use your AA, Q-W-E-R and when and how you flash.
If you know nothing how to win, what needs to be done, how your champ works and how the enemy interact with your champ, how good is mechanics if you dunno your output?
some people will say they know all this: yet, most of the time, they do something, it works but they dont know why. they would say: well we are stronger or blah blah.
Game knowledge is very low till mid plat. Most streamers will say that its decent only at masters.
wanna know the difference between bronze and masters? winning.
You should see what happens nightblue or imaqtpie when they arent in their role. They are MUCH worse mechanically, but they still win because they know their limits, what not to do and how to win with their team.