The constitutional argument is ridiculous; we've really been frog boiled into a wildly different understanding of executive power than even the most monarchical Founders imagined. Article II sections 2 and 3 are short and grant almost no powers. Practically all executive power outside stuff like appointments and pardons derives from this clause: "he shall take Care that the Laws be faithfully executed". Nearly all governmental power resides in Congress, on purpose, as it's the most democratically responsive yet least efficient of the branches. For example, a lot of people probably think either Commerce or Treasury mint coins. Nope, constitution says that's Congress. People think State, or maybe Commerce negotiates trade treaties; nope Congress again. Post Office? Congress.

What this means is that the President executes Congress' will. Reading the Constitution, there's just no argument for anything else. You have to dig into subsequent history, acts of Congress, and Supreme Court decisions to reach the justifications for the wild increase of presidential power. I'm not saying this is bad per se, just that the Constitution argument is absolutely bankrupt and ahistorical: there was zero appetite for a powerful executive in the Continental Congress.

That's what's so radical about this EO: it's antithetical to the very founding of the US where we rebelled against a king and ultimately adopted a constitution with a very weak executive. It supplants the will of Congress with the will of the executive, undermining the separation of powers, plenary powers, and the very underpinnings of our government.