As the quote you provided shows, labor mobility was rated as “not very tractable”, not “intractable”. Moreover, labor mobility was given that rating because it was judged to be politically infeasible, in light of the low popularity of even modest migration reform proposals, and not because we lack evidence from RCTs, or due to blindness to the mechanics of political change. So I think you are mischaracterizing what was said in the book.
As the quote you provided shows, labor mobility was rated as “not very tractable”, not “intractable”. Moreover, labor mobility was given that rating because it was judged to be politically infeasible, in light of the low popularity of even modest migration reform proposals, and not because we lack evidence from RCTs, or due to blindness to the mechanics of political change. So I think you are mischaracterizing what was said in the book.