As Lulushu said, some Plat players suck mechanically. I happen to be one of them so I can assure you it is true. When I get stuck top lane or bot lane, I lose the lane about 60% of the time. That does not mean I lose the game that often though. I have a win rate of about 55% with Jax. His laning phase is quite weak vs most top laners right now, but his late game is unparalleled if you know how to play it correctly. Knowing when to pressure, when to group, when to rotate, when to force objectives; all of this comes into play and none of it has to do with mechanics.
When snowdown came out, I lost 5 1v1 games in a row before I quite playing it. None of them had a rating as high as mine and yet I lost every single one pretty hard. Mechanics do not win as many games as game knowledge does.