You're talking about team defense rating, which is a product of Luka+4 guys on the court at the time of his minutes. That's not a great way of determining an individual rating as the other players are an important factor in how that is rated. The metric rewards steals, blocks and defensive rebounds, and penalizes you for being the closest defender when an opponent makes a shot. So if you can't stay in front of your man, and he drives into the paint for an easy shot, it's likely that a help defender will get hit with allowing the field goal because he's there trying to help on defense. I think we have seen some of that with Luka, which would hurt him on the eye test but not with statistics.
There's also a factor of who is taking those minutes in place of Luka. Given Dallas' depth, it's quite possible they are putting a worse defender in his place. If that's the case, then they should expect to see a disparity when he's off the floor.