I don't think KO has outperformed his draft slot at all. I think in a redraft he goes right around the same range if not a little worse (some of it is preference of course, but that draft while lacking top end talent aside from Giannis and maybe Gobert was pretty darn deep and even had the undrafted Covington and Dellavedova in it).
I don't think Smart goes 6 in a redraft either (Wiggins, Embiid, Jokic, Hood, Harris, Parker, Gordon, and Randle all likely go ahead of him and that doesn't account for LaVine, Capela, or Payton who all might as well).
Sullinger isn't even in the league anymore, and while he played very well for his draft slot while on the Celtics, Ainge got nothing for him when he left so it wasn't a salvageable draft pick (like say Fab Melo was).
It is personal preference but some of those are bogus. Choosing Hood, Parker, Gordon and Harris over Smart is simply bias. Parker has spent most of his career injured, Gordon has worse stats than Smart without any of the defense, Hood and Harris whilst competent are not the difference makers Smart is.
We got Horford using the cap space from renouncing Sullinger, that's not nothing. Just as renouncing KO to get Hayward is not nothing.
Finding 12 players to draft ahead of KO is not likely. Covington is not better than KO, nor is Delladova. You can hide behind preference if you want but stats don't back it up.