This is false. Considering winrate and MMR is specifically tied together, you can't necessarily have high MMR and low winrate. This is how the system is based:
If you have high MMR, your teammates will have a lower average MMR and the enemy team will have a higher average MMR. What this means is your team will be slightly worse than the enemy team where you're expected to lose. This is because your MMR is high, so your team has to have a lower MMR to compensate to balance the game out. However, if you're "good" enough to carry, you get more gains. It's not specifically set to "make" you lose, but rather to "test" how good you think you are.
Granted, this philosophy is poor but I think it's done to TRY to keep the game balanced and fair. If matchmaking was completely random, I think the game really would be a toss up, even more so than it is now. What Riot doesn't understand, however, is when your laners are completely outmatched and how devastating this is under any given standards. I had a Leblanc in game vs Ori who had 19 cs at 15minutes. There was nothing I could possibly do to close the skill gap of my mid laners because of how it's set up. Riot's been continuously removing the ability to solo carry, while keeping this philosophy making it neigh impossible to carry without completely relying on your teammates.