Leaving it open allows the courts to make decisions on "official acts", it doesn't leave it up to the President to decide. That's just silliness. Just like qualified immunity, it isn't the person acting that gets to decide how it applies, it is the courts that get to make those decisions.
There is nothing surprising in the SCOTUS ruling, it doesn't give "kinglike" powers to anyone. It simply means that if you are acting officially and within your legal authority (within the limits of the Constitution) then immunity can attach. If you are acting under a personal capacity, not so much... and instead of trying to decide for them they allowed the courts to make decisions regarding what is official and what is not.