Currency debasement is more monetary policy than fiscal policy I think. Either way I didn't read the GP comment as saying China can't do that, just that doing so is what gave them an edge long enough to gobble up a lot of our trade.
The same reason a sovereign state can't act unilaterally in any domain -- diplomacy. If your actions affect another (or even just inspire interest) then the other party may make decisions which affect your country.
Ironically I could see that idea being used in "hard authoritarian" style: a state could require that people MUST have distinct and uniquely registered faces to allow tracking via facial recognition.
Humans use soul juice to connect to the understandome. Machines can't connect to the understandome because of Gödels incompleteness, they can only make relationships between tokens. Not map them to reality like we can via magic.