Sound like a legitimate argument to me, that needs a serious refutation if you disagree with him.
Title 8 of the US Code is 1000 pages long. We have more than enough laws on the books about immigration.
What he's really saying is that Congress hasn't given him a law that he likes, so he's writing his own. Which even he admits isn't actually within his authority, but he's betting that nobody will be able to stop him.