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.
I don't think there is any team in the league that wouldn't want Gary Harris over Marcus Smart. It isn't preference with him. Hood got injured and hurt his play and stats last year, but he was trending as a much higher ceiling than Smart before the injury, but I'll give you preference on him.
What stats are worse for Gordon than Smart (aside from assists and steals - which makes sense since one is a guard and one is not). Gordon is a terrible shooter, but he is still better than Smart. Gordon scores more points than Smart. Gordon is a better rebounder. Gordon turns the ball over less and commits less fouls. That is both last year (for every one of those) and for their careers for almost everything. Smart is a better defender, but he is so bad on offense it hurts him a lot (and it isn't just his shooting it is his decision making on offense that is also bad). Smart is also nearly 2 years older than Gordon. I'm pretty confident that in a redraft Gordon goes ahead of Smart again.
Parker has been injured a lot, but he has a far higher ceiling than Smart, and thus would likely go ahead of him in a redraft.