I don't see anything in Steven's statement other than Marcus "was a good one", and that they will miss his leadership, requiring others to step up. Nothing here implies that he believes Smart was holding Tatum or Brown or the team in general back or that he was some obstacle that needed to be traded.
In 2021-22, we made it to the finals with Smart. Tatum was first team all NBA. I don't see that as Smart holding back anyone or the team in general.
In 2022-23, Brown made 2nd team all NBA, Tatum first, Smart didn't seem to hold anyone back that season either. The team had a meltdown vs MIA in the playoffs but if not for Tatum hurting his ankle in game 7, who knows. Are we saying the meltdown was Smart's fault?
It is a funny thing to try and construct, that the Celtics needed to trade Smart because he was too much of a leader. After Smart left, if anything Tatum regressed a little (ended lower in MVP voting) and Brown ended up not making all NBA. Pritchard was the player that made a leap after Smart left but that was due to role and opportunity I would say.
So I don't believe, based on Steven's statements that he felt he "needed to trade" Smart. If he did feel that way, I disagree, for all the reasons above. The team got better because we added Porzingis and Holiday, not because we subtracted Marcus Smart. There was no particular change in the trajectory of Tatum or Brown after Smart left. Smart was traded to get Porzingis, a trade I was all for. That improved the team tremendously, Porzingis changed so many things. Then we got Holiday and that put us over the top.