It's a relatively small sample size of 50 minutes. I'd also note that when KO is with the other starters, it's often at the end of games. Many of these times he's being switched in for offense, so he's only on the court when the Celtics have the ball, and thus a chance to gain points, but off the court when the Celtics are defending, and thus have a chance to give up points (it's either Amir or Marcus on the court in these situations). I think this artificially inflates his net rating, given the small amount of minutes overall.
That's not to say I think it's a bad lineup. I think it definitely has its strengths. I just think that purely going by net rating might ignore a confounding variable.