I don't think one has to believe the text is literally true in order to conclude that the argument made in this essay is specious. Both Michael and the Monsignor state that Catholic social teaching is grounded in the Hebrew scriptures and they make the mistake of attributing the Exodus account to the supposed exilic period. They're wrong on both counts. The Hebrew scriptures include a number of texts that speak of welcoming the stranger, but scholars agree they refer to fellow Hebrews. The same scriptures portray the Hebrews as conquering and annihilating neighboring peoples, which you correctly point out is neither historical nor deriving from the exilic period. 
So there is no basis for their argument, which is just fine with me. I think it's perfectly reasonable to state that Catholic social teaching has developed into what it is and we're all committed to it as the basis for our reflections on justice. Dragging in mythical figures and stories adds nothing to the argument.