There are 2 different types of MMR, one for Ranked and one for Normals. If someone just grinds ranked all the time and manages to get Platinum, then that's what they play against. But if they go play Normals, they will be matched up with Bronze and Unranked, because they never played Normals before, even though they are Platinum rank.
This is to help with the Smurf "problem". If there was only MMR for Ranked, then smurfs would go on a rampage destroying anyone and everything in their path. If there wasn't an MMR for normals, they would ALWAYS play against low tier players. If this guy is a Platinum/Diamond/Master tier player, no one would have fun playing against them.
I have a smurf myself and I was able to destroy new players with ease, until I noticed myself struggling a few games down the road. I was confused as to how a new player was so good until I checked op.gg and seen I as playing Gold and Plat players.
Since this is an automated system, it WILL have flaws. Someone may be truly a level 26 player, but has a lot of good games and now he's matched up with Silvers and Gold, players that are way out of his league. This is where he will lose a lot of games now. You can also DuoQ with someone that has a significantly lower MMR than yours. I'm Plat 3 and when I want to play with my Silver and Bronze friends, I go play normals and play against a bunch of other Bronze and Silver players.
It's just how it is.