So he got up to diamond by maining botlane, then got burned out and decided to try mid for awhile and lost a bunch of rank as a result, because he wasn't nearly as good mid as he was in bot.
I've had the same thing happen, albeit at much lower rank. It's maddening. I'm a decent support, but was getting burned out on it. I needed a change and switched for awhile and fell on my face. and yeah, I also had an embarrassing losing streak too. My rank seriously negatively adjusted to account for my newfound "skills."
I'd be more convinced if his 57% winrate on botlane turned into a 33% winrate in botlane and if his actual style changed, suggesting a different person (generally passive player becomes very aggressive player, goes from warding constantly to suddenly forgetting wards exist, though that particular one isn't so likely at the higher ranks; changes in demeanor in chat, etc)
Not saying he WASN'T boosted. Heck, he very well could have been. Just saying that a person could look at my own changes in playing habits during some time periods and conclude similar things (when I'm maining support, I'm climbing... but I don't wanna just support all the damn time and I get burned out)
Anecdotally, I've played with some truly unskilled silvers, even by comparison to bronze. Came to find out they were playing outside their previously mained position. And even by bronze standards, I'm a "trash" jungler but I sometimes jungle. I only do this in normals though (kinda like the guy you were talking about perhaps?). I can climb out of bronze on support, but I'm easily the equivalent of two leagues lower as jungle (I'm firmly Kleenex 5)
Honestly, I think this is a side-effect of being able to choose your positions now. Before, there was no guarantee and you had to be able maintain a certain standard of ability across all positions to hold steady or climb the elo ladder. Now, we can specialize.
