I never said that grinding games with griefers on both teams for a marginal increase in your MMR is *fun*. I said that theoretically, over a longer time, it's impossible to stay in the 'wrong' bracket because at some point, the disparity between your own skill level and the rest of the players around you will come into play.
If the person in this example is truly better than the bracket they are playing in, every time they play a match, the team that they are on will have an inherently higher chance to win simply because they have *one* player who is slightly better than the rest and is more or less guaranteed not to actively ruin the game. If you match up 10 random players from the same bracket, the odds are supposed to be 50/50. In your matches, however, there are only 9 random players + you. It's irrelevant that one in every couple of games is flat out unwinnable because of a single player who throws the game. Your team is guaranteed to always have at least one slot taken up by a player who presumably knows what they are doing (yourself), while the enemy team has no such guarantee.
Again, this is all under the assumption that the person really is better than their actual placement.