Yes the same number of active player playing in the game or given ranks. But its still extra accounts that are in the pool.
I.e. Create a Knockout Ladder for a Tournament. And lets reference Baseball stats for moment.
If you have 20 accounts that are normally in given pool half of them would be knocked out by second round. Then half again and again. This is normal. But say its 1 day wait time to get into round 2.
Now take the same ladder and add the same amount of smurfs from a higher tier playing in this pool with the 20 originals. So now thats 40 accounts. So instead of just getting done with round 1 in a day, its now taking round 1 twice as long to move out of across the board.
Now look at the stats this way. The original 20 accounts are like Little League, and the players have their stats punched out 0.255, 0.133, 0.198, etc. Nothing specific, just using numbers. And the playing field is a Little League regulation ball field (90-110ft) which is roughly as large as the entire dirt diamond area of a Major League field.
Then you add Major League (smurf accounts) and their stats look like 0.754, 0.899, 0.586, etc. Its drastically huge of a difference.
Now you cross reference these and compare, the difference is exponential where you are talking Little league players have like 10% chance of a home run, where as these Major League players would be hitting home runs 100% of the time, but their stats would be incalculable in comparison to the Little League players.
Now thats if none of them decided to throw or troll the lower leagues/ranks.
With Each Smurf account winning, that now diminishes those legit players that actually are in that rank trying to climb, down. So if they had an average of 15k damage, 20k heals, 50k block damage, etc… that has now all diminished and affected how their SR is calculated for their improved climb. in fact its probably crushed because its well below their average.
Now those with the smurf(s) on the winning team get a false boost in performance and will have a diminishing return sooner than later when the system thinks they need increased difficulty by including higher SR players to go against and/or include bad players on their team for them to carry.