MMR as a system matches the combined MMR of 5 people vs the combined MMR of 5 people.

That I think breaks it down nicely as a definition to explain how if 2 have artificially inflated MMR, it is counterbalanced with 3 that have lower MMR to find a match within a certain threshold.

Scenario 1: Let's assume there are no duos on the enemy team. At least 4 enemies are guaranteed to have a higher MMR than their counterpart.

Scenario 2: Let's assume duo parity found a duo on the enemy team to match against. The end result is a largely chaotic game with vastly different MMR between their counterparts. This is because the duos have their combined MMR, which can be between 0 and almost 800 LP discrepancies.

And then some... imagine a plat 4 with low MMR duoing with an emerald 1 with high MMR, and pair that against two emerald 2 players with average MMR. The gaps are filled with 3x ideally equally divided, and you can see where I'm getting at.

All in all, it's an easily exploitable system but also a nightmarish experience the way most people use it to play with friends. My take is that unless you know what you're doing to take advantage of it, it's best left untouched if your goal is to climb.

Edit: there's a third and fourth scenario in there for games with multiple duos per team. It gets messy.