I disagree with calling it an "ungame", but I don't necessarily disagree with the underlying criticism of that.
If you really look at Call of Duty it's basically a really high production value, very cinematic version of Duck Hunt. But Duck Hunt is clearly a game, and so is Call of Duty.
But I also think a lot of people are misunderstanding the criticism. He wasn't criticizing it for being a corridor shooter, he was criticizing it for spending so much time not even being that; for making you sit around and look at pretty things, or wait at a door while your squad yammers on.
Thank link Dean posted while I was writing illustrates it perfectly: you don't have to do anything in CoD, the game progresses with or without your involvement. (Granted, at this point I'm criticizing CoD in general, not MW3 specifically.) That's why RPS called it an ungame: because the game makes it feel as if the player is just watching the action, rather than a participant. It's a movie that allows you to control the viewing angle, and has an engaging, but entirely optional, shooting game built in.
I still enjoy playing Call of Duty a lot, but I recognize that that's a very valid criticism.