A lot of times, ETs are used as a symbol for unknown unknowns in a study. It is to add in a factor of complete chaos into the equation- something you can’t predict with certainty what it will be or its impact. It is like when you see a DOD study on an Alien Invasion or Zombies. If you can factor in the most outrageous situation where you can’t even guess at all the factors, you are better prepared for realistic events.
With economics, “ET” is a factor that would be difficult if not almost impossible to calculate the impact on the economic system.
Krugman is the fiscal “genius” who wants the government to fake an alien invasion so they can just take all the money they want.