thats how it works in league and valorant as well but since the mmr value doesnt equate 1:1 to a specific rank (i.e. 3500-3799 mmr = Archon 2) your rank isnt necessarily reflecting your mmr
in league you can for example be 3000 mmr and that could usually be equated to diamond 4 but if you have been on a huge winstreak starting from when you were low elo, your mmr has been climbing faster than your ingame points cus of said volatility correction. thats what he was talking about with separate systems. them being separate is only in the exception cases for new accounts and for people that are obviously misplaced rank wise in which case it places them in a higher rank lobby but their rank still hasnt caught up until they get to a that will stabilise their winrate around 50-60%