People need to be careful about queue types and what rank they're referring to.
It's quite possible for 5 players to be challenger in solo/duo ranked (with an mmr to match), bronze 5 in flex (with an mmr to match), and roughly the equivalent of silver 3 in normals mmr, and in every case if you looked at the wrong queues MMR, you'd find the matchmaking for them very confusing. And yet if you were careful and looked into it closely enough, you'd probably find that in every game they played, their teams average mmr for that queue was very similar to the enemy teams average mmr for that queue.
The fact that all 3 queue types have different mmrs, and further that mmrs aren't themselves visible tends to make for lots of "wtf how is this even remotely fair" moments, but barring huge match times, games are usually matchmade to be fair as far as the system knows. The on average part can also lead to some interesting matchups, especially if theres premades made up of a rather broad spectrum of elos.