You said it not me.
No, you're the one who said it, because the data doesn't mean what you think it means.
Makes sense. He spaces the floor by shooting aggressively. He creates additional offensive possessions through defensive plays and offensive rebounds. He creates fast break opportunities. He can make plays on the ball.
Generally speaking, Marcus' strengths play to the kind of things that guys like Irving, Hayward, and (near future) Tatum need out of their teammates. Smart taking on the most difficult defensive assignments allows them to save energy for offense and Smart acting as a primary initiator and talented passer allows them to focus on getting the ball in their most effective spots/situations. Sure, it'd be better if he were a better shooter, but his skills mesh well in lineups with star-level scorers.
Conversely, one of the better things that Rozier does is his ability to create off the dribble and score the ball. The problem is that if he's sharing the court with Kyrie, Hayward, and Tatum you don't actually
want Terry trying to be a primary scorer, and he's more of a defensive liability on the other end (not that he's bad, just that Smart's outstanding). It makes Rozier a better option for trying to carry a lineup of bench palyers and defensive specialists, but Smart's a better option for trying to hang with Golden State's starters.
So when Marcus goes 1 - 10 fg and 0 - 4 from the 3 it's a good thing?
Does it help this statistic when he has 5 or 6 turnovers?
"The best Celtics lineups generally feature Smart" isn't really a complicated idea to grasp. It may not make certain people happy, but it's facts.