This is a tricky argument. It doesn't follow occam's razor as it is a complex argument. They do do a convincing job, however. For most the article, they talk about how immigration is decreasing and Mexican's are actually emigrating more than immigrating. The author expresses to us in several instances that it is becoming less of an issue. They cite many of their fact claims, but they also leave many of the claims ungrounded. The author also describes underlying forces that affect immigration, such as the economy of Mexico and the US, as well as enforcement and changing policies. They go on to say that the debate is not over yet, however, and policy makers are still arguing. The tone of their writing suggests that there is no need to argue and that the debate should be over, but write at the end they throw a curveball. They start talking about illegal immigration and use strong claims saying its a problem. They say this lull in immigration should be used to strengthen defenses against illegal immigration before they are overwhelmed again.
This is an effective argument, because it sways those who may originally oppose it. It is a very interesting approach. They make you believe they are developing a different position, and right when you think you know what they are doing they switch it, leaving you consciously unsure of what they are advocating, but you are left with a message that they wanted you to have. They disguise the argument very well, which I believe was necessary because it is a hot button issue.