Brian Caplan was writing about the Charter Cities, which he likes as a second-best choice because:

The first-best solution to global poverty, therefore, is for the First World to allow much higher levels of immigration.  Unfortunately, despite its low absolute level (annual U.S. immigration is well under 1% of its population), immigration is already extremely unpopular.  For the foreseeable future, significantly more open borders – not to mention truly open borders – seem politically impossible.  The challenge, then, is to figure out a close substitute for free migration from the Third World to the First.  This is the challenge that Paul Romer’s increasingly influential “charter cities” proposal tries to meet.

Charter Cities does seem like a good second-best option, but everybody would benefit if we did the first-choice solution.