I saw the review then saw the film.
I've gotta say, I'm not sure Movie Bob and I were watching the same movie. The movie I watched did not have a "surprise twist" (and certainly a reference to Shyamalan seemed unwarranted).
The movie I saw seemed to be about how people get caught up in their own personal narratives. Over the course of the movie, the two main characters get slowly stripped of all the structure that they build up around themselves. The guy, a photographer, starts off hoping that this will be his big break. The girl, an engaged heiress who seems to be busy trying to "find herself", has been spending her time in Mexico, presumably trying to live some adventure. By the end of the movie, most of the personal conceits that come with their lives had been so broken down that they become able to actually see their own lives clearly.
I'm not sure whether it's because Movie Bob is American and I'm not, but the whole "illegal immigrant" thing did not seem to have anything to do with the movie's theme. It seemed more like a plot device - the infected zone is a place outside all normal experience; of nationality, of career, or personal desires (all the things that make up our personal dramas) and the border-crossers are just getting them there. In fact, it's particularly telling that the border-crossing party gets completely wiped out, leaving them behind and alone, that illegal immigration is NOT what the movie is getting at. It's also worth noting, Monsters is a BRITISH film, not an American film, and the US/Mexico border is significantly less of a focus for the British.
I think what the movie is actually getting at is exemplified by the aliens themselves. Throughout the entire movie, everyone is trying to kill the aliens. Everyone sees them as an invader, as something that needs to be destroyed - working the aliens into their own personal narrative as some massive invading enemy. But the main characters, having lost all their real attachments and having phoned their loved ones in calls that seemed very much to be farewells more than "I'll see you soons", are able to see that the aliens themselves don't even see people, let alone care about "invading". They're just living, and responding to people attacking them. They have no real motive or story. They just are.
With this realisation and the complete loss of all the personal bullshit they'd had at the start of the movie, the female lead then says "I don't want to go home".
And then "civilisation" swoops in to "rescue" them, but being completely obsessed with their own little story of heroism ("Da da daaa duh, da duh-duh dah duh, dun dun dun dun! It's my personal theme song! Everyone should have a theme song!"), they thunder off and get them all killed; too blind to see that they caused it in the first place.
It's not Cloverfield, and it's not District 9, I'll give Movie Bob that. But I definitely think he's missed the point of the movie... possibly in an ironic way, if my reading of the movie is correct.