Right, exactly what @Belisarius said. This kind of thing with fast food has come up before, with the famous in tact mcdonalds hamburger someone used to take around in a glass container, but the explanation was simple and you could (and some did) the experiment yourself with any burger. It’s not that it’s packed with odd chemicals that prevent life, it’s that it’s very cooked so that anything in it dies (the point of most cooking) and most of the juiciness/moisture is gone, or at least any left is taken away by the air after a couple hours or days.
McDonalds fries are generally of the crispy, oil fried type, which also requires removing most of the moisture during cooking. Molds and bacteria, like most known life, require water to live, and these aren’t providing much, and if sealed away well enough in a wall, even less can get to it to change that environment.
Probably still not what you want to eat at that point, unless there’s literally no other food, but not really all that strange or concerning either (not just based on being preserved).