No. There is an NBA rule. If he was with his original team, or he was on a team for four of the seven years, he’s eligible for a Super Max contract, which is 35% of the team’s salary. Because he is on a team for less time, he can only get 30% of the cap. It has nothing to do with the Lakers. If he was traded to any other team it would be the same.
Your math makes no sense. 35% is only 17% more than 30%, Doncic is making a third more with LA. But other than that, if I had written “partially exempt” instead of “exempt,” I would’ve pretty much stated what you wrote (minus the incorrect math).