It's based on a study called "Immigration, Offshoring and American Jobs,"? by Giovanni Peri [Email him]of the University of California, Davis, and two others, one of whom is a Professore at an Italian University, which means that, in effect, this offshoring study was partly offshored.
For George Borjas' comments on Peri's work, see here.
I am prepared to admit that immigrants do create jobs for a number of people:
And in terms of the free market, immigrant entrepreneurs frequently create jobs for other immigrants, usually their wives' relatives, in motels and convenience stores. What they do not do is create more jobs than they take, or provide a net benefit to the American nation.