Sure...
You posted the following:
Besides, POTUS is powerless in the face of Congress with respect to fiscal policy...
It's just a theoretical position.
The theoretical position can only be one of two things:
Trump's political position on S.A.L.T or the position of POTUS. Obviously the position of POTUS is not theoretical...it is real and created by the US Constitution.
The definition of theoretical is "concerned with or involving the theory of a subject or area of study rather than its practical application." His current position is his position...period. It is a real position. Only an idiot would think he could implement it unilaterally.