I'm assuming this was posted to draw a parallel with current U.S. international policy, such as tariffs and protectionist measures, which share some fundamental similarities with Brexit.
The paper just came out. I would assume they're posting it because it's interesting to a generally economically curious audience, and Brits in particular. No need for posting to be some targeted snipe at America.
Hopefully it leads to more investments in other places. That way the voters will eventually get what they want in a monkey’s paw way. Less immigration because the home countries get more investments and more opportunities.
Do you mean it turns out America was already great (despite still having some problems) and that’s why the whole world was flocking to it. In fact, it was so great that it was sucking up potential greatness from other countries.