Regarding this specific match:
1- Most of them are fresh out of tutorial, lvl 30 or less. I'd be shocked if they bought items or had loadouts.
2- They put one low-level smurf in the party so they can access the noob queue. That's how they pad their stats to hit 90% winrate, they only stomp noobs. It should match based on highest level in a party, not lowest level...
3- Most parties aren't manipulating the MM, they're fine. This particular group isn't friends, they recruit the highest level players (literally the highest levels in the game) from the LFG Discord with one of them as a smurf to pad their stats against noobs. Some of the high level players don't even know what is happening, but there's a core group of 3-5 players that do this for like eight hours a day, every day. Hence the winrates.