Indeed. That law and several others grant the president expanded powers to handle emergency situations. However, who determines what is an emergency and if the expanded powers are needed? In many cases, it is the president himself (or herself).
The underlying assumption is that the president would use such powers judiciously and that the expanded powers would enable emergency situations to be handled quickly since it would probably take more time for congress to respond and get things done.
The question now is: what if such powers are not used judiciously? What recourse is there?
> who determines what is an emergency and if the expanded powers are needed?
Congress. But so far they're letting Trump declare emergencies left and right. All of his tariffs are being enacted under an "emergency" to bypass Congress (since under non-emergency situations, Congress sets tariffs).
Had the Democrats won control of the House/Senate, a lot of this nonsense wouldn't be happening (or even if conventional Republicans had control, which is why Trump 1.0, when conventional GOP held the senate, Trump couldn't go off the rails as he has now).