Not all players who are good at CS / Not dying are smurfs. there are a lot of bronze, silver, gold players who are Mechanically good at the champions they play and are really strong and can snowball like crazy. However ... this is a team game, and so much more true now than it was back a few seasons ago. the reason many low elo players are stuck in lower elos is because they dont know how / when to press objectives, when to fight and when not to fight properly, where to rotate when an enemy is pushing many things like this factor into a win and loss.
Just yesterday i had a game where our team was stomping the enemy really hard ... but then for what ever reason no one really grouped, we coulda just pushed to win but instead we got caught out 1 by 1 and then loss. So while yes there are some insane players out there not all of them are diamond+ there are many mechanically great players stuck in lower elos. K/D/A isnt everything unless its the kpop group then its Life <3