...They might be drunk, but I think they're trying to say that you're matched based on your MMR only and ranks don't matter, so if one had a challenger MMR while bronze ranked, they'd still be playing with challengers (although you'd get tons of LP for wins and would almost certainly skip divisions/tiers when you won games).
In other words, a players rank(/tier) doesn't actually mean anything, since everyone in a given game is around the same MMR and so in theory around the same skill level. Whether that's a talented 'gold' in plat mmr game, a crappy 'diamond' in a plat mmr game, or a legit plat in a plat mmr game doesn't really matter, since they're in theory all plat skill level.