Your statistical performance is compared to the average player of your hero/rank/map. Smurfs throw that off, but so do throwers. This cancels out, because for a smurf to stay at low rank, they have to throw a lot of games. You only have to be better than the average game played to gain more than you lose. You do not need to be better than the smurfs when they are try harding.
For more detail, see How Competitive Matchmaking and Rating Works (Season 18) → Performane Modifier.
P.S.
Also, if we can assume that Blizzard developers are not idiots, they don’t include extremely good or poor performance in their statistical baselines (aka, outlier removal).