Theory that the soldiers in the movie aren't soldiers at all

So there is a theory that the soldiers were just marauders essentially who took the militaries weapons and kit. I disagree with this theory. For example one soldier calls another "corp".. Short for Corpral. Also when the infected attack the compound Sgt Farrell yells "stand to, stand to".. Only phrases real soldiers would know. Basically these soldiers were trained and knew what they were doing but the infection just turned them into horrible people. Apart from Sgt Farrell of course. One of my favourite characters.